perf(visible): crop MI-GAN around the mask so peak RAM is bounded by mark size

erase_migan fed the whole frame to the ONNX model, so peak RSS scaled with the
upload (~0.6 GB at 4 MP up to ~2.4 GB at 25 MP). Mirror erase_lama: crop a padded
region around the mask (pad = max(256, 2*bbox)), feed only that crop (at native
resolution -- MI-GAN accepts arbitrary dims, unlike LaMa's fixed 512 square), and
paste only masked pixels back. Peak RSS is now bounded by the mark size
(~0.6-0.9 GB), so a memory-tight host (a 1-2 GB web worker) can run MI-GAN on a
25 MP upload.

Fill quality is unchanged: verified by eye on real Gemini/Doubao marks plus a
ground-truth reconstruction sweep -- a tighter view if anything reduces the GAN's
hallucination of large background structure.

Extract the shared padded-crop-box math into _padded_crop_box (used by both
erase_lama and erase_migan).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/module-internals.md

@@ -139,7 +139,7 @@ The cost (mislabel ~8-33% of non-Gemini content as Gemini) outweighs the benefit
 
 `region_eraser.py` — universal region eraser (`erase` CLI) AND the shared fill backend behind `watermark_registry.fill` for the visible localize -> fill removal. `erase(image, boxes=|mask=, backend=)` accepts grayscale (2D) and RGBA (4-channel) inputs on **all** backends (each splits off any alpha plane and re-attaches it unchanged, and promotes grayscale to BGR): `boxes_to_mask` → one of three backends.
 - `cv2` (default, no deps): `cv2.inpaint`.
-- `migan` (extra `migan`, `andraniksargsyan/migan` ONNX, MIT, ~28 MB): `erase_migan`. The MI-GAN ONNX crops around the mask bbox and re-composites internally, so the FULL image is fed at native resolution; only masked pixels are pasted back. **Mask polarity is INVERTED** vs this package's 255-erase convention — the shipped ONNX wants 0=hole / 255=known, so `erase_migan` feeds `(mask<=127)*255`; feeding 255=hole regenerates the whole frame into stripes (corpus-validated 2026-07, cost hours to find). ~0.95 GB peak / ~0.19 s. This is the **preferred default fill** for the visible localize -> fill path.
+- `migan` (extra `migan`, `andraniksargsyan/migan` ONNX, MIT, ~28 MB): `erase_migan`. Like `erase_lama`, it crops a padded region around the mask (`pad = max(256, 2*bbox)`), feeds only that crop to the ONNX model, and pastes only masked pixels back — but since MI-GAN accepts arbitrary dims (unlike LaMa's fixed 512² square) the crop is fed at NATIVE resolution (no resize). This **bounds the ONNX working set by the mark size, not the image**: feeding the whole frame made peak RAM scale with the upload (~0.6 GB at 4 MP up to ~2.4 GB at 25 MP, measured 2026-07); cropping holds it roughly constant (~0.6-0.9 GB), so a memory-tight host (a 1-2 GB web worker) can run MI-GAN on a 25 MP upload. The crop does not degrade the fill — a small mark only needs local context, and on real marks the cropped fill is on par with / sometimes cleaner than the full-frame fill (a tighter view gives the GAN less room to hallucinate large background structure; verified by eye on real Gemini/Doubao marks + a ground-truth reconstruction sweep). **Mask polarity is INVERTED** vs this package's 255-erase convention — the shipped ONNX wants 0=hole / 255=known, so `erase_migan` feeds `(crop_mask<=127)*255`; feeding 255=hole regenerates the whole frame into stripes (corpus-validated 2026-07, cost hours to find). ~0.19 s. This is the **preferred default fill** for the visible localize -> fill path.
 - `lama` (extra `lama`, `Carve/LaMa-ONNX` Apache-2.0, ~200 MB): `erase_lama` crops a padded region around the mask, runs at LaMa's fixed 512² input, pastes only masked pixels back. Best quality but ~4.7 GB peak — explicit opt-in only, NOT auto-selected.
 Lazy `_get_{lama,migan}_session` singletons; `{lama,migan}_available()` guard the optional imports (both == onnxruntime present). Note both extras install the same onnxruntime, so the two `*_available()` checks are identical — the fill's `auto` backend therefore resolves to MI-GAN whenever onnxruntime is present, else cv2, and big-LaMa is reachable only by an explicit `lama` backend (`--backend lama` on `erase`, or the shared fill's `backend="lama"`).