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/banner-plan.json

@@ -388,6 +388,30 @@
     768
    ]
   },
+  "generating-subtitles-for-songs-without-timestamps": {
+   "sw": 1877,
+   "sh": 1060,
+   "left": 545,
+   "top": 70,
+   "fill": [
+    11,
+    18,
+    25
+   ],
+   "zoom": 1.38,
+   "silhouette": 0.47,
+   "cut": 0.0,
+   "band": [
+    743,
+    119,
+    1288,
+    635
+   ],
+   "source_size": [
+    1360,
+    768
+   ]
+  },
   "hardening-a-tor-onion-service": {
    "sw": 1952,
    "sh": 1102,
@@ -1755,6 +1779,30 @@
     1360,
     768
    ]
+  },
+  "yue2-on-a-12gb-gpu-songs-that-lost-their-endings": {
+   "sw": 1887,
+   "sh": 1066,
+   "left": 556,
+   "top": 77,
+   "fill": [
+    10,
+    14,
+    22
+   ],
+   "zoom": 1.387,
+   "silhouette": 0.47,
+   "cut": 0.0,
+   "band": [
+    747,
+    116,
+    1289,
+    643
+   ],
+   "source_size": [
+    1360,
+    768
+   ]
   }
  }
 }