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/sync_prompts_catalog.py

@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""sync_prompts_catalog.py — freeze the art inputs into the repo.
+
+The metaphors and captions are authored in the Ideogram 4 tool dir
+(`C:/AI/ideogram4/`), which is NOT in this repository. Without this script the
+repository could not reproduce a single frame: `prompts_catalog.json` would still
+describe the 21 shared metaphors that produced the near-duplicate v1-adjacent art.
+
+This copies the authoring state into the repo so git alone is enough:
+
+  scripts/og-gen/metaphors.json        slug -> the unique metaphor (72/72 distinct)
+  scripts/og-gen/captions/<slug>.json  the exact structured caption sent to the model
+  scripts/og-gen/prompts_catalog.json  same shape as before, refreshed from the above
+
+Usage:
+  python scripts/og-gen/sync_prompts_catalog.py                 # uses C:/AI/ideogram4
+  python scripts/og-gen/sync_prompts_catalog.py --tool-dir D:/other
+  python scripts/og-gen/sync_prompts_catalog.py --check          # report only, no writes
+"""
+from __future__ import annotations
+
+import argparse
+import importlib.util
+import json
+import shutil
+import sys
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parents[2]
+OG = REPO / "scripts" / "og-gen"
+
+
+def load_metaphors(tool_dir: Path) -> dict[str, str]:
+    src = tool_dir / "build_captions.py"
+    if not src.exists():
+        raise SystemExit(f"no build_captions.py under {tool_dir} (pass --tool-dir)")
+    spec = importlib.util.spec_from_file_location("build_captions", src)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+    return dict(mod.METAPHORS)
+
+
+def main() -> None:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--tool-dir", default="C:/AI/ideogram4")
+    ap.add_argument("--check", action="store_true", help="report only, write nothing")
+    args = ap.parse_args()
+
+    tool = Path(args.tool_dir)
+    metaphors = load_metaphors(tool)
+
+    posts = {p.stem for p in (REPO / "src" / "content" / "posts").glob("*.md")}
+    catalog = json.loads((OG / "prompts_catalog.json").read_text(encoding="utf-8"))
+    meta = json.loads((OG / "art-meta.json").read_text(encoding="utf-8"))
+
+    problems = []
+    if set(metaphors) != posts:
+        problems.append(f"slug mismatch: missing={sorted(posts - set(metaphors))[:4]} extra={sorted(set(metaphors) - posts)[:4]}")
+    if len(set(metaphors.values())) != len(metaphors):
+        problems.append(f"only {len(set(metaphors.values()))} distinct metaphors for {len(metaphors)} posts")
+
+    missing_captions = [s for s in metaphors if not (tool / "captions" / f"{s}.json").exists()]
+    if missing_captions:
+        problems.append(f"{len(missing_captions)} captions absent from {tool / 'captions'}")
+    missing_evidence = [s for s in metaphors if not meta.get(s, {}).get("evidence")]
+    if missing_evidence:
+        problems.append(f"{len(missing_evidence)} posts have no evidence line in art-meta.json")
+
+    print(f"posts {len(posts)} · metaphors {len(metaphors)} · distinct {len(set(metaphors.values()))} · catalog {len(catalog)}")
+    print("problems:", problems or "none")
+    if problems:
+        sys.exit(1)
+    if args.check:
+        print("--check: nothing written")
+        return
+
+    (OG / "metaphors.json").write_text(
+        json.dumps({k: metaphors[k] for k in sorted(metaphors)}, indent=2, ensure_ascii=False) + "\n",
+        encoding="utf-8",
+    )
+    dest = OG / "captions"
+    dest.mkdir(exist_ok=True)
+    for slug in metaphors:
+        shutil.copyfile(tool / "captions" / f"{slug}.json", dest / f"{slug}.json")
+
+    by_slug = {e["slug"]: e for e in catalog}
+    out = []
+    for slug in sorted(metaphors):
+        entry = by_slug.get(slug, {"slug": slug})
+        caption = json.loads((tool / "captions" / f"{slug}.json").read_text(encoding="utf-8"))
+        elems = caption.get("compositional_details", {}).get("elements", [])
+        entry["unique_elements"] = metaphors[slug]
+        entry["prompt"] = " ".join(e.get("desc", "") for e in elems).strip() or metaphors[slug]
+        entry["evidence_summary"] = meta.get(slug, {}).get("evidence", entry.get("evidence_summary", ""))
+        entry["caption"] = f"captions/{slug}.json"
+        out.append(entry)
+    (OG / "prompts_catalog.json").write_text(json.dumps(out, indent=2, ensure_ascii=False) + "\n", encoding="utf-8")
+    print(f"wrote metaphors.json, captions/ ({len(metaphors)} files), prompts_catalog.json")
+
+
+if __name__ == "__main__":
+    main()