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/og-gen/art-meta.json

@@ -221,5 +221,11 @@
  },
  "your-disk-full-alert-is-lying": {
   "evidence": "3% free = 500 GB · alert fires wrong way"
+ },
+ "yue2-on-a-12gb-gpu-songs-that-lost-their-endings": {
+  "evidence": "12 takes - all 240.000 s - cap now 300 s"
+ },
+ "generating-subtitles-for-songs-without-timestamps": {
+  "evidence": "18 songs - 342 lines - 0.96 anchor coverage"
  }
 }