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

scripts/og-gen/migrate_to_v2_assets.py

@@ -0,0 +1,73 @@
+#!/usr/bin/env python3
+"""migrate_to_v2_assets.py — point the content at the v2 artwork filenames.
+
+The v2 renderer (render_from_art.mjs) writes JPEG/WebP, not PNG:
+  public/banners/<slug>.jpg   (hero fallback; <picture> derives .webp)
+  public/og/<slug>.jpg        (EN share card)
+  public/og/zh-<slug>.jpg     (ZH share card)
+
+Until this migration runs, every page still points at the v1 PNG paths, so it has
+to happen in the SAME commit as the images or the site serves 404 heroes.
+
+Idempotent: it only rewrites values that still look like the v1 paths, keeps each
+file's quoting style, and prints what it changed. Run with --write to apply.
+
+  python scripts/og-gen/migrate_to_v2_assets.py           # report only
+  python scripts/og-gen/migrate_to_v2_assets.py --write
+"""
+from __future__ import annotations
+
+import argparse
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+POSTS = ROOT / "src" / "content" / "posts"
+ZH = POSTS / "zh"
+
+# ogImage: "/og/<slug>.png"  or  ogImage: /og/<slug>.png   (quotes optional)
+OG_RE = re.compile(r'^(ogImage:[ \t]*)(?P<q>"?)/og/(?P<name>[a-z0-9-]+)\.png(?P=q)[ \t]*$', re.M)
+BANNER_RE = re.compile(r'^(banner:[ \t]*)(?P<q>"?)/banners/(?P<name>[a-z0-9-]+)\.png(?P=q)[ \t]*$', re.M)
+
+
+def migrate(path: Path, zh: bool, write: bool) -> list[str]:
+    text = path.read_text(encoding="utf-8")
+    slug = path.stem
+    changes: list[str] = []
+
+    def og_sub(m: re.Match) -> str:
+        target = f"/og/zh-{slug}.jpg" if zh else f"/og/{slug}.jpg"
+        changes.append(f"ogImage {m.group(3)}.png -> {target}")
+        return f"{m.group(1)}{m.group('q')}{target}{m.group('q')}"
+
+    def banner_sub(m: re.Match) -> str:
+        target = f"/banners/{slug}.jpg"
+        changes.append(f"banner {m.group(3)}.png -> {target}")
+        return f"{m.group(1)}{m.group('q')}{target}{m.group('q')}"
+
+    new = OG_RE.sub(og_sub, text)
+    new = BANNER_RE.sub(banner_sub, new)
+    if new != text and write:
+        path.write_text(new, encoding="utf-8")
+    return changes
+
+
+def main() -> None:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--write", action="store_true")
+    args = ap.parse_args()
+
+    total = 0
+    files_changed = 0
+    for folder, zh in ((POSTS, False), (ZH, True)):
+        for p in sorted(folder.glob("*.md")):
+            ch = migrate(p, zh, args.write)
+            if ch:
+                files_changed += 1
+                total += len(ch)
+                print(f"{'WROTE' if args.write else 'would'} {p.relative_to(ROOT)}: " + "; ".join(ch))
+    print(f"\n{files_changed} files, {total} field(s) {'' if args.write else '(report only — pass --write)'}")
+
+
+if __name__ == "__main__":
+    main()