A songbook with melody, words and chords

// ═══ Postext Cookbook · Nº 164 · Songbook: melody, words under the notes, chords ═══════
// https://postext.dev/en/cookbook/songbook-melody-words-chords
// Code: MIT · Words and tunes: public domain (see the notes in each edition) · Prose: CC BY 4.0
// Fonts: Spectral, Spectral SC, Fraunces (SIL OFL 1.1); Leland (music) · Needs postext ≥ 1.54.0
import {
  buildDocumentWithFonts, renderPageToCanvas, initMusicEngine,
} from 'https://esm.sh/postext';
import { renderToPdf, decompressWoff2 } from 'https://esm.sh/postext-pdf';

const LANG = 'en'; // @lang: the language of the sample document ('en' | 'es')
const RECIPE = 'songbook-melody-words-chords';

// ─── 1 · Design ─────────────────────────────────────────────────────────────
const palette = { // every colour in the config links to one of these
  ink: '#211e1b', // text and notes: a warm near-black
  madder: '#8a3324', // the one accent: song numbers, the title page, credit lines
  tint: '#f1e7d8', // the subtitle on the madder ground (8.2:1)
  rule: '#cbbca6', // hairlines
  muted: '#675e55', // running heads, notes and the colophon (5.9:1 on the paper)
  paper: '#fbf8f1', // a cream stock
};
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' } })),
  // The engine's defaults link to 'main-color': point it at the accent, so nothing prints blue.
  { id: 'main-color', name: 'madder (defaults)', value: { hex: palette.madder, model: 'hex' } },
];
const [TEXT, LABEL, DISPLAY] = ['Spectral', 'Spectral SC', 'Fraunces'];
const LEAD = 14; // pt: the body's leading
const OUTER = 15; // mm: the outer margin, where the running heads and folios align

// #region answer: the score style: a 5 mm staff in Leland, the words in the book's face
// A song is an ```abc fence in the Markdown: the melody, its chords written "G" before the note
// they fall on, and the first verse on a w: line under it. The look is set here, once.
const scoreStyle = {
  staffSize: mm(5.5), // a songbook rastral; the words under it come out at about 8 pt
  musicFont: 'Leland', // lighter noteheads than Bravura, close to Spectral's colour
  textFont: TEXT, // words and chord names in the text face, not the engraver's Times
  spacing: 1, // the engraver's own: five or six bars of 3/4 to a system on this measure
  barNumbers: 'none', // a table of singers counts verses, not bars
  systemGap: mm(4.5), // room for the chord names between one system's words and the next
  marginTop: pt(4),
  marginBottom: pt(LEAD),
  color: col('ink'),
  minSystemsAtBreak: 2, // never a lone system at a page foot or head
};
// Two habits of Verovio's ABC reader shape the w: lines. Every sung note takes the next
// syllable in order: the `_` and `*` holds are read as nothing, so the second note of a