Chuleta de anclajes: un cartel de elementos encadenados

// ═══ Postext Cookbook · Nº 029 · Anchoring cheat sheet: a poster built from chained elements ═══
// https://postext.dev/en/cookbook/anchoring-cheat-sheet
// Code: MIT · Text: original (CC BY 4.0) · Drawings: generated in code (CC BY 4.0)
// Fonts: Funnel Display, Funnel Sans, Martian Mono (SIL OFL 1.1) · Needs postext ≥ 1.4.1
// An A3 poster whose elements hang from the bleed, the page, their slot or one another, never
// from coordinates; the second sheet is the same poster with each element framed and tagged.
import {
  buildDocument, renderPageToCanvas, clearMeasurementCache, registerResourceImage,
} from 'https://esm.sh/postext';

const LANG = 'es'; // @lang: the language of the sample document ('en' | 'es')
const RECIPE = 'anchoring-cheat-sheet';

// ─── 1 · Design ─────────────────────────────────────────────────────────────
const palette = {
  ink: '#0b0b0c', // text, and the Moon
  sky: '#1b3bff', // the sky, the foot band and the dates
  sun: '#ffe53b', // the Sun and the kicker
  paper: '#ffffff', // type on the sky
  muted: '#5c5f66', // speakers and the colophon
  guide: '#ff2d9b', // frames and tags on the construction sheet
};
// col(id): a palette-linked colour that also carries its hex, because 1.4.1 paints design
// elements from the hex (gotcha: palette-skips-designs).
const col = (id) => ({ hex: palette[id], model: 'hex', paletteId: id });
const colorPalette = [
  ...Object.entries(palette).map(([id, hex]) => ({ id, name: id, value: { hex, model: 'hex' } })),
  { id: 'main-color', name: 'sky (defaults)', value: { hex: palette.sky, model: 'hex' } },
];

const A3 = { width: 297, height: 420 }; // mm, the trim
const MARGIN = 18; // mm at the top and sides: the body's edges, and the header container's
const FOOT = 30; // mm, the bottom margin: the footer container and its band
const BLEED = 0; // mm; 3 for the printer, which switches on page.cutLines below
// at(): a placement. Offsets are distances from the anchor point, never page coordinates.
const at = (to, edge, x = 0, y = 0, size) => ({ anchor: { to, edge },
  offset: { x: mm(x), y: mm(y) }, ...(size && { size }) });
const wide = (w) => ({ width: mm(w) }); // fixed, so a text is as wide as its column
const square = (d) => ({ width: mm(d), height: mm(d) });
// Design text is centred and cut with '…' by default (gotcha: overflow-ellipsis-default).
const text = (id, content, face, placement) => ({ kind: 'text', id, content, overflow: 'wrap',
  align: 'left', ...face, placement });
const face = (fontFamily, size, fontWeight, colour, more) => ({ fontFamily, fontSize: pt(size),
  fontWeight, color: col(colour), ...more });
const caps = (tracking) => ({ letterSpacing: pt(tracking), textTransform: 'uppercase' });
const FACE = { // lineHeight: a multiple, never pt() (gotcha: design-lineheight-multiple)
  kicker: face('Martian Mono', 12, 500, 'sun', caps(1.8)),
  title: face('Funnel Display', 120, 800, 'paper', { lineHeight: 0.86 }),
  deck: face('Funnel Sans', 22, 400, 'paper', { lineHeight: 1.22 }),
  day: face('Funnel Display', 84, 800, 'sky', { lineHeight: 0.9 }),
  talk: face('Funnel Sans', 19, 600, 'ink', { lineHeight: 1.14 }),
  who: face('Funnel Sans', 12.5, 400, 'muted', { lineHeight: 1.3, italic: true }),
  foot: face('Martian Mono', 9, 500, 'paper', caps(0.9)),
};

// #region answer: two elements pinned to frames of the sheet, every other one to an element
const SKY = 272; // mm from the trim's top edge to the horizon, the sky's lower edge
const INSET = BLEED + MARGIN; // from the bleed's edge to the text's edge
const AIR = 14; // mm between the standfirst and the horizon
const poster = () => ({ enabled: true, slot: { elements: [ // array order is paint order
  // 'bleed' and 'page' are frames of the sheet; 'container' would be this heading's own box.
  { kind: 'box', id: 'sky', style: { backgroundColor: col('sky') }, // no width: fills the bleed
    placement: at('bleed', 'top-left', 0, 0, { height: mm(BLEED + SKY) }) },
  ...eclipse(), // the Sun hangs from the trim's corner, the Moon and the corona from the Sun
  // The headline stands on the horizon and grows upwards: 'above' puts an element's
  // bottom-left corner on the top-left corner of its anchor, so a longer title lifts the kicker.
  text('deck', '{attr.deck}', FACE.deck, { ...at('#sky', 'align-bottom', INSET, -AIR),
    size: { maxWidth: mm(153) } }), // shrink-wraps its lines, but wraps at 153 mm
  text('title', '{titleText}', FACE.title, at('#deck', 'above', 0, -9)), // breaks at the \\
  text('kicker', '{attr.kicker}', FACE.kicker, at('#title', 'above', 0, -7)),
  ...programme(), // four columns chained off the horizon
] } }); // hook-up: headings.levels[0] = { span: 'page', breakBefore, advancedDesign: poster() }
// {titleText} is the heading's text; each {attr.*} is written on its line in content.md.
// #endregion

// #region eclipse: the Sun pinned to the trim's corner, the Moon and the corona to the Sun
const SUN = 150; // mm across; the Moon is drawn the same size
const CORONA = 300; // mm, the square picture of the corona
const HALO = (CORONA - SUN) / 2; // how far the corona reaches past the Sun on every side
const BITE = { x: 2, y: 2.5 }; // mm the Moon sits off the Sun: a sliver is left at upper left
const disc = (id, colour, placement) => ({ kind: 'box', id, placement: { ...placement,
  size: square(SUN) }, style: { backgroundColor: col(colour), borderRadius: mm(SUN / 2) } });
const eclipse = () => [
  { kind: 'image', id: 'corona', resourceId: 'corona', // listed before the discs: painted behind
    placement: at('#sun', 'align-top', -HALO, -HALO, square(CORONA)) },
  disc('sun', 'sun', at('page', 'top-right', 30, 25)), // 30 mm of it past the right edge
  disc('moon', 'ink', at('#sun', 'align-top', BITE.x, BITE.y)),
];
// #endregion

// #region programme: four evenings in fixed-width columns, chained right-of and below
const GUT = 7; // mm between the evenings, and between the two columns of text under them
const COL = (A3.width - 2 * MARGIN - 3 * GUT) / 4; // four columns across the text width: 60 mm
const DROP = 12; // mm from the horizon to the dates
const programme = () => [1, 2, 3, 4].flatMap((n) => [
  text(`day${n}`, `{attr.d${n}}`, FACE.day, n === 1
    ? at('#sky', 'below', INSET, DROP, wide(COL)) // the first date hangs from the horizon
    : at(`#day${n - 1}`, 'right-of', GUT, 0, wide(COL))), // the others from the one before
  text(`talk${n}`, `{attr.t${n}}`, FACE.talk, at(`#day${n}`, 'below', 0, 3, wide(COL))),
  text(`who${n}`, `{attr.s${n}}`, FACE.who, at(`#talk${n}`, 'below', 0, 2.5, wide(COL))),
]);
// #endregion

// #region footer: a band and the club's mark in the footer slot, which prints on every page
const MARK = 10; // mm, the club's mark
const PAD = (FOOT - MARK) / 2; // centres the mark in the bottom margin
const markTall = { height: mm(MARK) }; // as tall as the mark: the line is centred on it
const footer = { elements: [
  { kind: 'box', id: 'band', style: { backgroundColor: col('sky') }, // the bottom margin, bled
    placement: at('bleed', 'bottom-left', 0, 0, { height: mm(BLEED + FOOT) }) },
  // The container runs from the text down to the trim: 'bottom-*' counts up, 'top-*' down.
  { kind: 'image', id: 'mark', resourceId: 'mark',
    placement: at('container', 'bottom-left', 0, -PAD, square(MARK)) },
  text('club', '{attr.club}', FACE.foot, at('#mark', 'right-of', 3, 0, markTall)),
  text('free', '{attr.free}', FACE.foot, at('container', 'top-right', 0, PAD, markTall)),
] };
// #endregion

// #region guides: the same slots, each element framed and tagged with its own placement
const GUIDE = { borderColor: col('guide'), borderWidth: pt(1) };
const signed = ({ value }) => `${value < 0 ? '−' : '+'}${Math.abs(value)}`; // all offsets in mm
const describe = ({ id, placement: { anchor: { to, edge }, offset = {} } }) => [`#${id}`,
  `${edge} ${to}`, // the two words of anchor: { to, edge }
  ['x', 'y'].filter((k) => offset[k]?.value).map((k) => `${k} ${signed(offset[k])}`).join(' '),
].filter(Boolean).join(' · ');
// A border never changes an element's size (padding does), so each frame traces its box.
// An image or a rounded box gets a square frame of its size, hung from its top-left corner.
const frame = (el) => (el.kind === 'text' ? [{ ...el, box: { ...el.box, ...GUIDE } }]
  : el.kind === 'box' && !el.style.borderRadius ? [{ ...el, style: { ...el.style, ...GUIDE } }]
    : [el, { kind: 'box', id: `${el.id}-box`, style: GUIDE,
      placement: at(`#${el.id}`, 'align-top', 0, 0, el.placement.size) }]);
const tag = (el, [edge, x = 0, y = 0, note]) => text(`${el.id}-tag`, note ?? describe(el),
  face('Martian Mono', 8.5, 500, 'ink', { box: { backgroundColor: col('guide'),
    padding: { top: pt(1.6), bottom: pt(1.2), left: pt(3), right: pt(3) } } }),
  at(`#${el.id}`, edge, x, y));
const guides = (elements, tags) => [...elements.flatMap(frame),
  ...elements.filter((el) => tags[el.id]).map((el) => tag(el, tags[el.id]))];
const container = (id) => ({ kind: 'box', id, style: GUIDE, // the slot's own box, drawn
  placement: at('container', 'top-left', 0, 0, { width: 'fill', height: 'fill' }) });
const TAGS = { // where each tag sits against its element: [edge, x, y, text]
  sky: ['align-bottom', INSET, -3], corona: ['align-bottom', 90, -2], sun: ['above'],
  moon: ['below', 20, 2], kicker: ['above'], title: ['above'], deck: ['above'],
  day1: ['above', 0, -6], day2: ['above'], who1: ['below'],
  mark: ['above', 0, -2], club: ['above', 69, -2], free: ['above', -30, -2],
  header: ['align-bottom', 0, -2, t({ en: 'header container', es: 'contenedor de la cabecera' })],
  footer: ['align-bottom', 0, -2, t({ en: 'footer container', es: 'contenedor del pie' })],
};
// #endregion

const LEAD = 16; // body leading in pt
const config = () => ({ // a factory: configs are cached by identity (gotcha: config-cache-identity)
  colorPalette,
  page: { width: mm(A3.width), height: mm(A3.height), dpi: 150, // 150 dpi is for the screen
    cutLines: { enabled: BLEED > 0, bleed: mm(BLEED) },
    margins: { top: mm(MARGIN), bottom: mm(FOOT), left: mm(MARGIN), right: mm(MARGIN) } },
  layout: { layoutType: 'double', gutterWidth: mm(GUT) }, // the text columns under the evenings
  bodyText: { fontFamily: 'Funnel Sans', fontSize: pt(12), lineHeight: pt(LEAD), color: col('ink'),
    boldColor: col('ink'), italicColor: col('ink'), textAlign: 'left', firstLineIndent: pt(0),
    paragraphSpacing: true },
  // The hidden title is still measured, so it needs a face FONTS loads.
  headings: { fontFamily: 'Funnel Display', fontWeight: 800,
    levels: [{ level: 1, span: 'page', advancedDesign: poster(),
      breakBefore: { enabled: true, parity: 'any' } }] }, // gotcha: headings-drop-h1-break
  // {style="guides"}: the same design framed and tagged, and a header and footer of its own.
  headingStyles: [{ id: 'guides',
    advancedDesign: { enabled: true, slot: { elements: guides(poster().slot.elements, TAGS) } },
    header: { elements: guides([container('header')], TAGS) },
    footer: { elements: guides([...footer.elements, container('footer')], TAGS) } }],
  paragraphStyles: [{ id: 'colophon', fontFamily: 'Martian Mono', fontSize: pt(8),
    lineHeight: pt(LEAD * 0.75), color: col('muted') }],
  header: { elements: [] }, // the poster has none; the guides page draws the empty container
  footer,
});

// ─── 2 · Content ────────────────────────────────────────────────────────────
const markdown = String.raw`# Vuelve\\la\\sombra {kicker="Jueves de abril de 2027 · 19:30" deck="Cuatro charlas para preparar el eclipse total del 2 de agosto de 2027, cuando la sombra de la Luna cruce el Estrecho." d1="8" t1="Por qué la Luna tapa justo el Sol" s1="Marta Iribarren, astrónoma" d2="15" t2="Por dónde cruzará la sombra" s2="Óscar Beltrán, meteorólogo" d3="22" t3="Lo que nos cuenta la corona" s3="Lucía Ferrándiz, física" d4="29" t4="Mirar al Sol sin quemarse los ojos" s4="Tomás Rey, optometrista" club="Círculo Umbra · astrónomos aficionados desde 1987" free="Entrada libre · sin reserva"}
Muestra en Markdown · 14 líneas · content.es.md

Cada charla empieza a las 19:30 y dura alrededor de una hora, con preguntas. La sede del Círculo Umbra está en la calle del Faro, 14; las puertas se abren a las 19:00, la entrada es libre y no hay reserva, así que las 120 butacas son para quien llegue antes.

El 29 de abril cada asistente se lleva unas gafas de eclipse conformes a la norma ISO 12312-2: úsalas en cada fase parcial y quítatelas solo mientras el Sol esté cubierto del todo. La mañana del eclipse, el Círculo monta telescopios con filtro en el muelle.

# Vuelve\\la\\sombra {style="guides" kicker="Jueves de abril de 2027 · 19:30" deck="Cuatro charlas para preparar el eclipse total del 2 de agosto de 2027, cuando la sombra de la Luna cruce el Estrecho." d1="8" t1="Por qué la Luna tapa justo el Sol" s1="Marta Iribarren, astrónoma" d2="15" t2="Por dónde cruzará la sombra" s2="Óscar Beltrán, meteorólogo" d3="22" t3="Lo que nos cuenta la corona" s3="Lucía Ferrándiz, física" d4="29" t4="Mirar al Sol sin quemarse los ojos" s4="Tomás Rey, optometrista" club="Círculo Umbra · astrónomos aficionados desde 1987" free="Entrada libre · sin reserva"}

**Cómo leer esta hoja.** Cada marco rosa traza la caja de un elemento, y una etiqueta da su id, el borde por el que se ancla, a qué se ancla y el desplazamiento en milímetros, si lo hay. Por encima del pie, solo dos elementos cuelgan de la hoja misma: el cielo, de la sangre, y el Sol, de la página, que es el corte. Todos los demás cuelgan de otro elemento, así que un título más largo sube el antetítulo y las cuatro tardes siguen al horizonte.

Los marcos de los márgenes superior e inferior son los contenedores de la cabecera y del pie. El del pie va del texto al corte, así que las anclas de abajo cuentan hacia arriba desde el corte y las de arriba, hacia abajo desde el texto.

:::paragraphs{style="colophon"}
Tipos: Funnel Display, Funnel Sans y Martian Mono (SIL OFL) · Texto y dibujos: originales, CC BY 4.0 · El Círculo Umbra y sus ponentes son ficticios.
:::
`; // content.<lang>.md, inlined by the Cookbook

// #region art: the corona and the club's mark, drawn in code, and their resources
function mulberry32(seed) { // a seeded generator: the same corona on every run
  return () => {
    seed = (seed + 0x6d2b79f5) | 0;
    let t = Math.imul(seed ^ (seed >>> 15), 1 | seed);
    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}
function coronaSvg() { // a glow, then 14 streamers, 70 polar plumes and 160 fine rays
  const rand = mulberry32(20270802);
  const R = (500 * SUN) / CORONA; // the Sun's radius in the 1000-unit square
  const TILT = Math.PI / 4; // the Sun's equator: long streamers go up-left, clear of the kicker
  const TYPE = [2.2, 3.05]; // radians from the centre where the type is: rays there stop at 9 mm
  const p = (r, a) => `${(500 + r * Math.cos(a)).toFixed(1)} ${(500 + r * Math.sin(a)).toFixed(1)}`;
  const ray = (a, full, half, opacity) => { // a petal from the limb, tapered to a point
    const turn = ((a % (2 * Math.PI)) + 2 * Math.PI) % (2 * Math.PI);
    const length = turn > TYPE[0] && turn < TYPE[1] ? Math.min(full, R * 0.12) : full;
    const bend = (rand() - 0.5) * 0.06;
    return `<path d="M${p(R * 0.97, a - half)}C${p(R + length * 0.35, a - half * 1.1)} `
      + `${p(R + length * 0.7, a + bend - half * 0.25)} ${p(R + length, a + bend)}`
      + `C${p(R + length * 0.7, a + bend + half * 0.25)} ${p(R + length * 0.35, a + half * 1.1)} `
      + `${p(R * 0.97, a + half)}Z" fill="${palette.paper}" fill-opacity="${opacity.toFixed(3)}"/>`;
  };
  const glow = Array.from({ length: 28 }, (_, i) => `<circle cx="500" cy="500" `
    + `r="${(R * (1.01 + i * 0.022)).toFixed(1)}" fill="${palette.paper}" fill-opacity="0.022"/>`)
    .reverse().join('');
  const out = [];
  for (let i = 0; i < 14; i++) { // helmet streamers, two fans across the equator
    const a = TILT + (i % 2) * Math.PI + (rand() - 0.5) * 1.1;
    out.push(ray(a, R * (0.55 + 0.45 * rand()), 0.1 + 0.14 * rand(), 0.06 + 0.06 * rand()));
  }
  for (let i = 0; i < 70; i++) { // polar plumes, shorter and thinner than the streamers
    const a = TILT + Math.PI / 2 + (i % 2) * Math.PI + (rand() - 0.5) * 1.3;
    out.push(ray(a, R * (0.18 + 0.3 * rand()), 0.008 + 0.012 * rand(), 0.1 + 0.12 * rand()));
  }
  for (let i = 0; i < 160; i++) { // fine rays all round
    const a = (i / 160) * 2 * Math.PI + (rand() - 0.5) * 0.04;
    out.push(ray(a, R * (0.12 + 0.3 * rand()), 0.006 + 0.014 * rand(), 0.05 + 0.07 * rand()));
  }
  return `<svg xmlns="http://www.w3.org/2000/svg" width="1500" height="1500" `
    + `viewBox="0 0 1000 1000">${glow}${out.join('')}</svg>`;
}
function markSvg() { // the Umbra Circle's mark: a ring round an eclipsed Sun
  return `<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200" viewBox="0 0 100 100">`
    + `<circle cx="50" cy="50" r="45" fill="none" stroke="${palette.paper}" stroke-width="6"/>`
    + `<circle cx="50" cy="50" r="27" fill="${palette.sun}"/>`
    + `<circle cx="56" cy="45" r="27" fill="${palette.sky}"/></svg>`;
}
const svg = (id, size, altText) => ({ id, typeId: 'figure', kind: 'svg', altText,
  svg: { fileId: `${id}.svg`, width: size, height: size }, createdAt: 0, updatedAt: 0 });
// Image elements draw resources. Nothing cites them, so neither is placed or numbered as a figure.
const resources = [
  svg('corona', 1500, t({ en: 'The solar corona round the eclipsed Sun',
    es: 'La corona solar alrededor del Sol eclipsado' })),
  svg('mark', 200, t({ en: 'The Umbra Circle’s mark', es: 'El emblema del Círculo Umbra' })),
];
// #endregion

// ─── 3 · Fonts ──────────────────────────────────────────────────────────────
// Every face the pages paint, loaded before the first build (gotcha: fonts-first).
const FONTS = {
  'Funnel Display': ['800'],
  'Funnel Sans': ['400', '400i', '600', '700'],
  'Martian Mono': ['400', '500'],
};

// ─── 4 · Build & show ───────────────────────────────────────────────────────
await loadSvg('corona.svg', coronaSvg());
await loadSvg('mark.svg', markSvg());
await loadFonts(FONTS, markdown);
// pageIndexOffset 1 makes the poster a verso, so the viewer sets the two sheets side by side.
const content = { markdown, resources, continuation: { pageIndexOffset: 1 } };
const doc = await buildWithFonts(() => buildDocument(content, config()), markdown);
showPages(doc, { title: t({ en: 'Anchoring cheat sheet', es: 'Chuleta de anclajes' }) });

Kit · core, fonts, viewer, images: igual en todas las recetas · 270 líneas
// ─── Kit ── helpers shared by every Cookbook recipe · postext.dev/cookbook ─────

// ─── Kit · core v1 ── the same in every recipe · postext.dev/cookbook ─────────
function mm(value) { return { value, unit: 'mm' }; }
function pt(value) { return { value, unit: 'pt' }; }
function em(value) { return { value, unit: 'em' }; }
/** The sample language's string: t({ en: 'Figure', es: 'Figura' }). */
function t(strings) { return strings[LANG] ?? Object.values(strings)[0]; }
/** A file in this recipe's assets folder, served from the Postext repo by jsDelivr. */
function asset(file) { return `https://cdn.jsdelivr.net/gh/drnachio/postext@main/cookbook/${RECIPE}/assets/${file}`; }

// ─── Kit · fonts v1 ── the same in every recipe · postext.dev/cookbook ────────
// Postext measures text with the faces the browser has loaded, and caches the
// widths, so every face must be ready before the first build. Faces come from
// Fontsource: the same static files the PDF embeds, so screen and PDF agree.

/** faces = { 'Family Name': ['400', '400i', '700'] }. `text` is the sample:
 *  letters beyond Latin-1 (č, ł, ő…) also load the latin-ext files. With
 *  `optional`, a face Fontsource does not ship is skipped instead of failing.
 *  Resolves to the number of faces added. */
async function loadFonts(faces, text = '', { optional = false } = {}) {
  kitStatus('Loading fonts…');
  const ranges = {
    latin: 'U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,'
      + 'U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD',
    'latin-ext': 'U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,'
      + 'U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF',
  };
  const subsets = /[Ā-˿Ḁ-ỿ]/.test(text) ? ['latin', 'latin-ext'] : ['latin'];
  const jobs = [];
  let added = 0;
  for (const [family, specs] of Object.entries(faces)) {
    const id = fontsourceId(family);
    const meta = optional ? await fontsourceMeta(family) : null;
    for (const spec of new Set(specs)) {
      const weight = parseInt(spec, 10);
      const style = spec.endsWith('i') ? 'italic' : 'normal';
      if (hasFace(family, weight, style)) continue;
      if (optional && !(meta?.weights.includes(weight) && meta.styles.includes(style))) continue;
      for (const subset of subsets) {
        const url = `https://cdn.jsdelivr.net/npm/@fontsource/${id}@5/files/${id}-${subset}-${weight}-${style}.woff2`;
        const face = new FontFace(family, `url(${url}) format('woff2')`,
          { weight: String(weight), style, unicodeRange: ranges[subset] });
        jobs.push(face.load().then((ready) => { document.fonts.add(ready); added++; }, () => {
          if (subset === 'latin' && !optional) throw new Error(`Fontsource has no ${family} ${weight} ${style}`);
        }));
      }
    }
  }
  await Promise.all(jobs).catch((error) => { kitFail(error); throw error; });
  return added;
}

/** Runs `build` (a buildDocument or buildBundle call) and checks the faces
 *  the pages use. A regular face missing from FONTS is loaded with a warning;
 *  bold and italic variants are loaded when the family ships them. Then the
 *  measurement caches are cleared and the build runs again. */
async function buildWithFonts(build, text = '') {
  const tried = new Set();
  for (let round = 0; round < 3; round++) {
    kitStatus('Laying out…');
    await new Promise(requestAnimationFrame);          // let the status paint first
    const result = await Promise.resolve().then(build).catch((error) => { kitFail(error); throw error; });
    const wanted = { base: {}, variants: {} };
    for (const { font, base } of [result].flat().flatMap(fontStringsOf)) {
      const { family, weight, style } = parseFont(font);
      const key = `${family}|${weight}|${style}`;
      if (tried.has(key) || hasFace(family, weight, style)) continue;
      tried.add(key);
      (wanted[base ? 'base' : 'variants'][family] ??= []).push(`${weight}${style === 'italic' ? 'i' : ''}`);
    }
    if (Object.keys(wanted.base).length) {
      console.warn(`[cookbook] FONTS does not list ${JSON.stringify(wanted.base)}: loading them.`);
    }
    const added = await loadFonts(wanted.base, text) + await loadFonts(wanted.variants, text, { optional: true });
    if (added === 0) return result;
    clearMeasurementCache();
  }
  throw new Error('The fonts did not settle after three builds.');
}

/** Every font string of the layout. `base` marks a block's own face; its
 *  bold, italic and bold-italic variants are listed whether or not used. */
function fontStringsOf(doc) {
  const found = new Map();
  const walk = (node) => {
    if (!node || typeof node !== 'object') return;
    if (Array.isArray(node)) { node.forEach(walk); return; }
    for (const [key, value] of Object.entries(node)) {
      if (typeof value === 'string' && /fontString$/i.test(key)) {
        found.set(value, found.get(value) || key === 'fontString');
      } else if (value && typeof value === 'object') walk(value);
    }
  };
  walk(doc.pages);
  walk(doc.blocks);
  return [...found].map(([font, base]) => ({ font, base }));
}

/** '700 37.5px Open Sans' / 'italic 400 13px "Source Serif 4"' → { family, weight, style }.
 *  A string with no weight ('95.8px Young Serif', from a design text) is 400. */
function parseFont(font) {
  const m = /^(?:(italic|oblique)\s+)?(?:small-caps\s+)?(?:(\d+|bold|normal)\s+)?[\d.]+px\s+(.+)$/.exec(font.trim());
  if (!m) throw new Error(`Unexpected font string: ${font}`);
  const weight = m[2] === 'bold' ? 700 : !m[2] || m[2] === 'normal' ? 400 : Number(m[2]);
  return { family: m[3].replace(/^["']|["']$/g, ''), weight, style: m[1] ? 'italic' : 'normal' };
}

/** True when a loaded FontFace covers exactly this family, weight and style
 *  (document.fonts.check() is also true for families nobody declared). */
function hasFace(family, weight, style) {
  for (const face of document.fonts) {
    if (face.status !== 'loaded' || face.style !== style) continue;
    if (face.family.replace(/^["']|["']$/g, '') !== family) continue;
    const [low, high = low] = face.weight.split(' ').map(Number);
    if (weight >= low && weight <= high) return true;
  }
  return false;
}

/** Fontsource's id for a family: 'Source Serif 4' → 'source-serif-4'. */
function fontsourceId(family) { return family.toLowerCase().replace(/\s+/g, '-'); }

/** The weights and styles a family ships ({ weights: [400, 700], styles: ['normal', 'italic'] }), or null. */
function fontsourceMeta(family) {
  fontsourceMeta.cache ??= new Map();
  const id = fontsourceId(family);
  if (!fontsourceMeta.cache.has(id)) {
    fontsourceMeta.cache.set(id, fetch(`https://api.fontsource.org/v1/fonts/${id}`)
      .then((res) => (res.ok ? res.json() : null), () => null));
  }
  return fontsourceMeta.cache.get(id);
}

// ─── Kit · viewer v1 ── the same in every recipe · postext.dev/cookbook ───────
/** Shows the pages as facing spreads on a dark desk: the first page is a
 *  recto on its own, then verso | recto pairs, as in a bound book. Pages
 *  are painted when they scroll near the screen. */
function showPages(docs, { title, width = 460 } = {}) {
  const root = viewer(title);
  const pages = [docs].flat().flatMap((doc) =>
    doc.pages.map((page) => ({ doc, page, n: (doc.pageIndexOffset ?? 0) + page.index })));
  const spreads = [];
  let verso = null;
  for (const p of pages) {
    if (p.n % 2 === 1) { if (verso) spreads.push([verso, null]); verso = p; }
    else { spreads.push([verso, p]); verso = null; }
  }
  if (verso) spreads.push([verso, null]);
  const density = Math.min(window.devicePixelRatio || 1, 2);
  showPages.painter?.disconnect();
  const painter = new IntersectionObserver((entries) => {
    for (const { isIntersecting, target } of entries) {
      if (!isIntersecting) continue;
      painter.unobserve(target);
      const { doc, page } = target.postext;
      renderPageToCanvas(page, doc, target, { scale: (width * density) / page.width });
    }
  }, { rootMargin: '800px' });
  showPages.painter = painter;
  root.replaceChildren(...spreads.map((pair) => {
    const spread = document.createElement('div');
    spread.className = 'pt-spread';
    for (const p of pair) {
      const figure = document.createElement('figure');
      if (p) {
        const label = p.page.pageLabel || String(p.n + 1);
        const canvas = document.createElement('canvas');
        canvas.postext = p;
        canvas.style.aspectRatio = `${p.page.width} / ${p.page.height}`;
        canvas.setAttribute('role', 'img');
        canvas.setAttribute('aria-label', `Page ${label}`);
        const folio = document.createElement('figcaption');
        folio.textContent = label;
        figure.append(canvas, folio);
        painter.observe(canvas);
      } else figure.className = 'pt-blank';
      spread.append(figure);
    }
    return spread;
  }));
  kitStatus(`${pages.length} ${pages.length === 1 ? 'page' : 'pages'}`);
  document.documentElement.dataset.postext = 'ready';
  return pages.length;
}

/** The desk, the bar and the error reporting, created once. */
function viewer(title) {
  if (!document.getElementById('pt-kit')) {
    document.head.insertAdjacentHTML('beforeend', `<style id="pt-kit">
      :root { color-scheme: dark; }
      body { margin: 0; background: #0e1014; color: #b9bcc4; font: 13px/1.45 system-ui, sans-serif; }
      #pt-bar { position: sticky; top: 0; z-index: 1; display: flex; flex-wrap: wrap; align-items: center;
        gap: 6px 16px; padding: 10px 16px; background: rgb(14 16 20 / .92); backdrop-filter: blur(6px);
        border-bottom: 1px solid #23262d; }
      #pt-bar strong { color: #f4f1ea; font-weight: 600; }
      #pt-actions { display: flex; gap: 12px; margin-left: auto; }
      #pt-actions a, #pt-actions button { color: #d8a21a; font: inherit; background: none; border: 0; padding: 0; cursor: pointer; }
      #pages { display: grid; justify-items: center; gap: 48px; padding: 32px 16px 72px; }
      .pt-spread { display: flex; }
      .pt-spread figure { margin: 0; width: min(460px, 44vw); }
      .pt-spread canvas { display: block; width: 100%; background: #fff;
        box-shadow: 0 1px 2px rgb(0 0 0 / .5), 0 22px 44px -16px rgb(0 0 0 / .8); }
      .pt-spread figure:first-child canvas { box-shadow: inset -14px 0 14px -14px rgb(0 0 0 / .18), 0 1px 2px rgb(0 0 0 / .5), 0 22px 44px -16px rgb(0 0 0 / .8); }
      .pt-spread figcaption { margin-top: 10px; text-align: center; font: 600 10px/1 system-ui, sans-serif;
        letter-spacing: .18em; text-transform: uppercase; color: #6c7079; }
      .pt-blank { visibility: hidden; }
      @media (max-width: 760px) {
        .pt-spread { flex-direction: column; gap: 32px; }
        .pt-spread figure { width: min(460px, 92vw); }
        .pt-blank { display: none; }
      }
    </style>`);
    document.body.insertAdjacentHTML('afterbegin',
      '<header id="pt-bar"><strong id="pt-title"></strong><span id="pt-status" role="status"></span><span id="pt-actions"></span></header>');
    document.getElementById('pt-title').textContent = document.title || 'Postext';
    addEventListener('error', (event) => kitFail(event.error ?? event.message));
    addEventListener('unhandledrejection', (event) => kitFail(event.reason));
  }
  if (title) document.getElementById('pt-title').textContent = title;
  return document.getElementById('pages')
    ?? document.body.appendChild(Object.assign(document.createElement('main'), { id: 'pages' }));
}

function kitStatus(text) {
  viewer();
  document.getElementById('pt-status').textContent = text;
}

function kitFail(error) {
  document.documentElement.dataset.postext = 'error';
  kitStatus(`Error: ${error?.message ?? error}`);
}

// ─── Kit · images v1 ── recipes with pictures · postext.dev/cookbook ──────────
/** Registers a photo or PNG for the canvas and keeps its bytes for the PDF.
 *  fetch → ImageBitmap never taints the canvas (a plain cross-origin <img> would). */
async function loadImage(fileId, url) {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`Image not found (${res.status}): ${url}`);
  const bytes = new Uint8Array(await res.arrayBuffer());
  registerResourceImage(fileId, await createImageBitmap(new Blob([bytes])));
  (loadImage.bytes ??= new Map()).set(fileId, bytes);
}

/** Registers SVG markup (drawn in code, or fetched) as a vector image. */
async function loadSvg(fileId, svg) {
  const img = new Image();
  img.src = `data:image/svg+xml;charset=utf-8,${encodeURIComponent(svg)}`;
  await img.decode();
  registerResourceImage(fileId, img);
  (loadImage.bytes ??= new Map()).set(fileId, new TextEncoder().encode(svg));
}

/** renderToPdf({ resourceBytes: imageBytes }) */
function imageBytes(fileId) { return loadImage.bytes?.get(fileId); }

/** renderToHtml({ resourceImageUrl: imageUrl }) */
function imageUrl(fileId) {
  const bytes = imageBytes(fileId);
  if (!bytes) return undefined;
  imageUrl.urls ??= new Map();
  if (!imageUrl.urls.has(fileId)) {
    const type = /\.svg$/i.test(fileId) ? 'image/svg+xml' : /\.png$/i.test(fileId) ? 'image/png' : 'image/jpeg';
    imageUrl.urls.set(fileId, URL.createObjectURL(new Blob([bytes], { type })));
  }
  return imageUrl.urls.get(fileId);
}

// ─── /Kit ───────────────────────────────────────────────────────────────────────