fix(art): give both 2026-10-20 posts their artwork, and gate the build on it

The YuE2 and subtitles posts shipped text-only: render_from_art.mjs only READS
ogImage/banner, and after the 2026-10-19 bulk migration no script had to write
them, so both pages advertised /og-default.jpg and rendered no hero banner.

- new metaphors (in build_captions.py, frozen here via sync_prompts_catalog.py),
  captions, art-meta evidence, banner plan and the five derived files per slug
- frontmatter: ogImage + banner in EN and ZH for both posts
- scripts/check-assets.mjs is now step 1 of npm run build, so CI fails when a
  published post lacks a language-correct og card, banner, banner .webp or
  400/800 list thumb. It reported 16 problems on the unfixed tree, all here.

scripts/check-assets.mjs

@@ -0,0 +1,134 @@
+#!/usr/bin/env node
+/**
+ * Publish gate: every published post must carry its own artwork.
+ *
+ * Why this exists: the "one art, four targets" pipeline writes five files per
+ * slug (banner .jpg/.webp, EN card, ZH card, two list thumbs) but the two
+ * frontmatter fields that *point* at them (`ogImage`, `banner`) are added by
+ * hand — they were bulk-migrated for the first 72 posts and nothing wrote them
+ * for posts added later. Two posts shipped on 2026-10-20 with an empty
+ * `ogImage`, so their pages advertised /og-default.jpg and rendered no hero
+ * banner, and every related-post card showed the generic default. Nothing in
+ * the build noticed. This script is that notice.
+ *
+ * Runs as the first step of `npm run build`, so a post that is text-complete
+ * but not image-complete cannot reach nginx.
+ *
+ * Rules checked per non-draft post:
+ *   - `ogImage` present, language-correct (EN /og/<slug>.jpg, ZH /og/zh-<slug>.jpg)
+ *   - `banner` present (/banners/<slug>.jpg) — the same banner serves both languages
+ *   - every referenced file exists in public/, plus the derived banner .webp and
+ *     the two list thumbs the post list and related grid link to
+ *
+ * Usage: node scripts/check-assets.mjs   (exit 1 with a list on any problem)
+ */
+import { readFileSync, existsSync, readdirSync } from 'node:fs';
+import { basename, join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const ROOT = dirname(dirname(fileURLToPath(import.meta.url)));
+const POSTS = join(ROOT, 'src', 'content', 'posts');
+const PUBLIC = join(ROOT, 'public');
+
+/** Minimal frontmatter reader: top-level `key: value` scalars only, which is all we need. */
+function frontmatter(file) {
+  const text = readFileSync(file, 'utf8').replace(/^\uFEFF/, '');
+  const m = text.match(/^---\r?\n([\s\S]*?)\r?\n---/);
+  if (!m) return null;
+  const out = {};
+  for (const line of m[1].split(/\r?\n/)) {
+    const kv = line.match(/^([A-Za-z_][\w-]*):\s*(.*)$/);
+    if (!kv) continue;
+    let value = kv[2].trim();
+    if (
+      (value.startsWith('"') && value.endsWith('"')) ||
+      (value.startsWith("'") && value.endsWith("'"))
+    ) {
+      value = value.slice(1, -1);
+    }
+    out[kv[1]] = value;
+  }
+  return out;
+}
+
+const problems = [];
+const notes = [];
+
+/** Resolve a site-absolute path (/og/x.jpg) to a file on disk under public/. */
+const publicFile = (urlPath) => join(PUBLIC, urlPath.replace(/^\//, ''));
+
+function requireFile(urlPath, label) {
+  if (!existsSync(publicFile(urlPath))) problems.push(`${label}: missing file ${urlPath}`);
+}
+
+const files = [
+  ...readdirSync(POSTS)
+    .filter((f) => f.endsWith('.md'))
+    .map((f) => ({ dir: POSTS, file: f, lang: 'en' })),
+  ...(existsSync(join(POSTS, 'zh'))
+    ? readdirSync(join(POSTS, 'zh'))
+        .filter((f) => f.endsWith('.md'))
+        .map((f) => ({ dir: join(POSTS, 'zh'), file: f, lang: 'zh' }))
+    : []),
+];
+
+let checked = 0;
+for (const { dir, file, lang } of files) {
+  const path = join(dir, file);
+  const slug = basename(file, '.md');
+  const fm = frontmatter(path);
+  if (!fm) {
+    problems.push(`${file}: no frontmatter block`);
+    continue;
+  }
+  if (fm.draft === 'true') continue;
+  checked++;
+
+  const where = lang === 'zh' ? `zh/${file}` : file;
+  const expectedOg = lang === 'zh' ? `/og/zh-${slug}.jpg` : `/og/${slug}.jpg`;
+  const expectedBanner = `/banners/${slug}.jpg`;
+
+  if (!fm.ogImage) {
+    problems.push(`${where}: no og:image in frontmatter (expected ogImage: "${expectedOg}")`);
+  } else if (fm.ogImage !== expectedOg) {
+    problems.push(`${where}: ogImage is "${fm.ogImage}", expected "${expectedOg}"`);
+  } else {
+    requireFile(fm.ogImage, where);
+  }
+
+  if (!fm.banner) {
+    problems.push(`${where}: no banner in frontmatter (expected banner: "${expectedBanner}")`);
+  } else if (fm.banner !== expectedBanner) {
+    problems.push(`${where}: banner is "${fm.banner}", expected "${expectedBanner}"`);
+  } else {
+    requireFile(fm.banner, where);
+    requireFile(fm.banner.replace(/\.(png|jpe?g)$/, '.webp'), where);
+  }
+
+  // The post list serves 400w/800w WebP thumbs keyed by BASE slug (a ZH card is
+  // zh-<slug>.jpg while its thumb is <slug>-<w>.webp), so check both languages here.
+  for (const w of [400, 800]) requireFile(`/og/thumb/${slug}-${w}.webp`, where);
+}
+
+// Every post should have a twin, and both should agree it exists.
+const enSlugs = new Set(files.filter((f) => f.lang === 'en').map((f) => basename(f.file, '.md')));
+const zhSlugs = new Set(files.filter((f) => f.lang === 'zh').map((f) => basename(f.file, '.md')));
+for (const s of enSlugs) if (!zhSlugs.has(s)) notes.push(`no ZH twin for ${s}`);
+for (const s of zhSlugs) if (!enSlugs.has(s)) notes.push(`no EN twin for zh/${s}`);
+
+if (problems.length) {
+  console.error(`assets: ${problems.length} problem(s) across ${checked} published posts\n`);
+  for (const p of problems) console.error(`  ✗ ${p}`);
+  if (notes.length) {
+    console.error('');
+    for (const n of notes) console.error(`  ! ${n}`);
+  }
+  console.error(
+    '\nFix: cd C:/AI/ideogram4 && uv run --with pillow python post_art.py --slug <slug> --metaphor "..." --evidence "..."' +
+      '\nthen add ogImage + banner to BOTH language files (see docs/design-guide.md, image policy).'
+  );
+  process.exit(1);
+}
+
+console.log(`assets: ${checked} published posts OK (og card + banner + webp + 2 thumbs each)`);
+for (const n of notes) console.log(`  ! ${n}`);