feat(art): replace all 72 posts' artwork with the v2 pipeline

Every post now carries one unique local-ComfyUI/Ideogram 4 base frame, composited
into EN + ZH share cards, a banner and list thumbs.

- the shipped prompts_catalog held only 21 distinct metaphors for 72 posts (16
  shared one visual), so all 72 were rewritten as unique object+state metaphors
  and 10 frames re-rolled after OCR found pseudo-text rendered at legible size
- render_from_art.mjs emits JPEG by default (558 KB PNG -> 84 KB mozjpeg q85);
  the ZH 1200x630 card did not exist before this batch
- 288 frontmatter fields migrated; the 144 v1 PNGs are gone
- PostList's thumb helper keys off the base slug (a ZH card is zh-<slug>.jpg while
  the thumb is <slug>-800.webp, which 404'd the whole ZH listing)
- [...slug].astro's <picture> regex accepts .jpg
- the repo now carries the art inputs (metaphors.json, captions/<slug>.json,
  refreshed prompts_catalog.json) so a frame is reproducible from git alone

src/pages/posts/[...slug].astro

@@ -109,10 +109,10 @@ const jsonLd = {
 
     {post.data.banner && (
       <figure class="article-banner">
-        {/* WebP for the page (42% of the PNG bytes); the PNG stays for og:image
+        {/* WebP for the page (42% of the PNG bytes); the JPEG/PNG stays for og:image
             because social scrapers do not reliably render WebP share cards. */}
         <picture>
-          <source srcset={post.data.banner.replace(/\.png$/, '.webp')} type="image/webp" />
+          <source srcset={post.data.banner.replace(/\.(png|jpe?g)$/, '.webp')} type="image/webp" />
           <img src={post.data.banner} alt="" loading="lazy" decoding="async" width="1600" height="900" />
         </picture>
       </figure>