A Japanese drill with the answers upside down

// ═══ Postext Cookbook · Nº 172 · A Japanese drill with the answers upside down ═══
// https://postext.dev/en/cookbook/japanese-drill-upside-down-answers
// Code: MIT · Text: original (CC BY 4.0) · Pictures: generated with diffusion models
// Fonts: Klee One, Zen Maru Gothic (SIL OFL 1.1) · Needs postext ≥ 1.59.0
import {
  buildDocument, withLoadedFonts, renderPageToCanvas, registerResourceImage,
  loadVerticalAlternates,
} from 'https://esm.sh/postext';
import { renderToPdf, decompressWoff2 } from 'https://esm.sh/postext-pdf';

const LANG = 'ja'; // @lang: the language of the frame; the drill is Japanese in all
const RECIPE = 'japanese-drill-upside-down-answers';

// ─── 1 · Design ─────────────────────────────────────────────────────────────
// #region palette: a drill book's blue, a warm yellow and a pale sky for the band
const palette = {
  ink: '#23272c', // text
  blue: '#2a6cb0', // the accent: exercise numbers, the title band's rule, the score box
  sun: '#f2c14e', // the subject badge
  tint: '#eaf2fb', // the title band
  rule: '#a8bfd8', // writing lines and boxes
  muted: '#69717b', // folios, the name line, the upside-down answers
  paper: '#ffffff',
};
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: 'accent (defaults)', value: { hex: palette.blue, model: 'hex' } },
];
// #endregion
// 教科書体 for the text a child copies from, a round gothic for the labels.
const [TEXT, ROUND] = ['Klee One', 'Zen Maru Gothic'];

// #region answer: the answers printed upside down under each exercise
// `placement: 'inverted'` sets each exercise's key right after it in a box turned 180°: the
// pupil sees where the answers are without reading them, and turns the book to check. The
// key's words come from the exercise (the options' katakana, ○ and ×, the operations), so
// the Markdown holds nothing twice.
const drill = {
  id: 'drill',
  number: { shape: 'square', fontFamily: TEXT, fontWeight: 600, background: col('blue') },
  instruction: { fontFamily: ROUND, bold: false },
  options: { layout: 'row' }, // ア イ ウ side by side (the default labels in Japanese)
  trueFalse: { look: 'bracket' }, // （　） for ○ or ×
  blank: { look: 'box', width: 'answer', color: col('rule') }, // □ to write in
  area: { kind: 'lines', color: col('rule') },
  grid: { rule: col('rule') },
  solutions: { placement: 'inverted', box: 'kotae' },