feat(visible): capture-less AI生成 pill (#54), inpaint fallback, MI-GAN backend (#56)

- Add the Jimeng-basic top-left "AI生成" pill as a CAPTURE-LESS mark
  (pill_engine.py): synthetic-silhouette edge-NCC detect + inpaint-only removal.
  Gated in remove_auto_marks: kept only when Jimeng is confirmed (TC260 metadata
  OR the bottom-right "★ 即梦AI" wordmark fired -- the wordmark keeps recall on
  metadata-STRIPPED uploads) AND Doubao did not fire.
- Add an inpaint-fallback removal path + MI-GAN ONNX backend (migan extra, MIT,
  ~28 MB / ~1 GB peak -- droplet-friendly) alongside big-LaMa. New
  --method auto|reverse-alpha|inpaint (shared across visible/all/batch) and
  erase --backend migan; footprint_mask on each engine.
- auto is deterministic: reverse-alpha for capture marks (recovers exact pixels,
  lighter -- measured cleaner than MI-GAN on structured backgrounds) and inpaint
  only for the capture-less pill.
- --mark auto now removes EVERY detected mark in one pass (remove_auto_marks),
  so a Jimeng-basic image's top-left pill AND bottom-right wordmark both clear.
- Bump 0.12.1 -> 0.13.0.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/render_pill_silhouette.py

@@ -0,0 +1,60 @@
+"""Render the SYNTHETIC 'AI生成' pill silhouette asset (data-safe, font-rendered).
+
+The Jimeng-basic TC260 visible label is a rounded pill with 'AI生成' in the TOP-LEFT
+corner (issue #54). Unlike the reverse-alpha marks, this is a CAPTURE-LESS mark:
+the committed asset is a font-rendered binary SILHOUETTE (mark shape only, zero photo
+content), used ONLY to (a) detect the pill by edge-NCC in the top-left corner and
+(b) build the inpaint mask. It is NOT an alpha map -- removal quality comes from the
+inpaint backend (MI-GAN/cv2), so the silhouette need not be pixel-accurate, and the
+synthetic render keeps corpus/user content out of the tracked repo (data-safety).
+
+Detection was calibrated on the retained local corpus (61 real positives + jimeng
+negatives): edge-NCC threshold ~0.22 in the top-left ROI. Re-run to regenerate the
+asset:  uv run python scripts/render_pill_silhouette.py
+
+Requires a CJK font (macOS STHeiti by default); the asset itself is committed, so this
+script only runs when regenerating it (never in CI).
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import numpy as np
+from PIL import Image, ImageDraw, ImageFont
+
+_ASSET = Path(__file__).resolve().parents[1] / "src" / "remove_ai_watermarks" / "assets" / "jimeng_pill.png"
+_FONT = "/System/Library/Fonts/STHeiti Medium.ttc"
+
+
+def render_silhouette(w: int = 320) -> np.ndarray:
+    """Rounded-pill outline + 'AI生成' text as a binary silhouette (255 = mark)."""
+    h = int(w * 0.5)
+    im = Image.new("L", (w, h), 0)
+    d = ImageDraw.Draw(im)
+    pad = int(w * 0.03)
+    r = (h - 2 * pad) // 3
+    d.rounded_rectangle([pad, pad, w - pad, h - pad], radius=r, outline=255, width=max(2, w // 90))
+    fsz = int(h * 0.42)
+    font = ImageFont.truetype(_FONT, fsz)
+    txt = "AI生成"
+    tb = d.textbbox((0, 0), txt, font=font)
+    tw, th = tb[2] - tb[0], tb[3] - tb[1]
+    d.text(((w - tw) // 2 - tb[0], (h - th) // 2 - tb[1]), txt, font=font, fill=255)
+    return np.array(im)
+
+
+def main() -> None:
+    try:
+        sil = render_silhouette()
+    except OSError as e:
+        print(f"Font not found ({e}); install a CJK font or edit _FONT.", file=sys.stderr)
+        raise SystemExit(1) from e
+    _ASSET.parent.mkdir(parents=True, exist_ok=True)
+    Image.fromarray(sil).save(_ASSET)
+    print(f"wrote {_ASSET}  ({sil.shape[1]}x{sil.shape[0]})")
+
+
+if __name__ == "__main__":
+    main()