feat(visible): Doubao text-mark removal + universal region eraser

Add deterministic, CPU-only removal of the visible Doubao "豆包AI生成" mark and
a position-agnostic region eraser for any other visible watermark/logo.

- doubao_engine.py: locate (geometry, scales with width) + polarity-aware
  white-top-hat glyph mask + cv2 inpaint; coverage-gated detection and a
  dense-text safety guard. No GPU, ~30ms.
- region_eraser.py + `erase` command: inpaint arbitrary --region box(es).
  Default cv2 backend (no deps); optional big-LaMa via onnxruntime (`lama`
  extra, Carve/LaMa-ONNX, model downloaded on first use, never bundled).
- cli `visible --mark auto|gemini|doubao`: auto routes by detector confidence.
- tests for both engines; seed previously-unseeded CLI image fixtures to stop
  the Doubao detector flaking on random corners.
- .gitignore: doubao_capture/{seeds,captures} scratch (alpha-map calibration).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

data/doubao_capture/README.md

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+# Doubao visible watermark capture
+
+Goal: capture the Doubao "豆包AI生成" visible watermark over known flat backgrounds so we can
+build a per-pixel alpha map and a reverse-alpha-blend remover, the same way the Gemini sparkle
+engine works (`src/remove_ai_watermarks/gemini_engine.py`).
+
+## What we already know (verified from prior art, 2026-05-26)
+
+- Blend model: **alpha compositing with a white logo** `watermarked = a*logo + (1-a)*original`,
+  `logo = (255,255,255)`. Inversion: `original = (watermarked - a*logo) / (1-a)`.
+  Confirmed by two independent sources (an open-source remover's algorithm doc + aiwatermarkremover.dev,
+  both say "alpha map"). One commercial blog (pixelcleanai) claims "screen blend" instead; the gray
+  capture below settles it empirically.
+- Position: **bottom-right corner**, small margins (right ~8-20px, bottom ~5px), scales with image size.
+  Confirmed by our sample `data/samples/doubao-1.png` (2048x2048) plus three sources.
+- Size **scales with resolution**. Third-party numbers (~90x18 at <=1024, ~180x40 at >1024) are
+  approximate and calibrated for ~1024-1280 outputs; at 2048 the strip is much larger. A shipped
+  third-party alpha map is only 120x20, too small for our 2K/4K target -> capture fresh.
+- In practice clean inversion leaves residue on textured backgrounds, so the remover pairs the alpha
+  map with inpainting (our Gemini engine already does gradient-masked inpainting for residual edges).
+
+## Use doubao.com specifically
+
+The "豆包AI生成" mark is Doubao's. Jimeng / Dreamina use a different mark. Generate on doubao.com so
+the captured template matches our target.
+
+## How to capture (image-edit path, most reliable)
+
+For each seed in `seeds/`:
+
+1. Open Doubao image generation, use the image-edit / reference mode, upload the seed.
+2. Prompt (Chinese preferred):
+   `请完全按照原图重新生成这张图片，保持完全一致，不要添加或修改任何内容`
+   (English: `Recreate this image exactly as it is, keep it identical, do not add or change anything`)
+3. Download the ORIGINAL output file (not a screenshot). Do not crop / edit / re-save.
+
+Prior art confirms uploading a pure-black image and letting Doubao stamp it works.
+
+If edit mode is unavailable and text-to-image refuses a solid color, fall back to generating 10-12
+normal-content images at one fixed resolution; the mark is the only constant across them and can be
+extracted by per-pixel min/median.
+
+## What to capture (priority top to bottom)
+
+| Aspect | black | white | gray128 | why |
+|--------|-------|-------|---------|-----|
+| 1:1    | 3     | 1     | 1       | primary alpha map + confirm the stamp is pixel-identical across runs + settle blend mode |
+| 16:9   | 2     | 1     | 1       | anchor rule in landscape |
+| 9:16   | 2     | 1     | 1       | anchor rule in portrait |
+| 4:3, 3:4 | 1 each | -   | -       | optional, refines anchor rule |
+
+- 3 blacks on 1:1: if the first two are byte-identical in the watermark region, the third is optional.
+- gray128 is the blend-mode test: predict the gray result from the black capture under alpha vs screen;
+  whichever matches the real gray output is the true blend.
+- If the UI offers multiple output resolutions (1K / 2K / 4K), capture one black per resolution on 1:1 -
+  needed to learn how the watermark scales.
+- Also grab 3-5 normal-content images on 1:1 for end-to-end removal validation.
+
+## Hygiene
+
+- Original download, never a screenshot. PNG preferred; if Doubao only gives JPEG, note it.
+- No crop / edit / re-save. Default settings, watermark left ON.
+
+## Naming, drop into `captures/`
+
+```
+doubao_black_1x1_1.png
+doubao_white_1x1_1.png
+doubao_gray128_1x1_1.png
+doubao_black_16x9_1.png
+doubao_content_1x1_1.png
+```
+
+## Also report back
+
+1. Which resolutions and aspect ratios the Doubao UI actually offers.
+2. Whether there is a watermark on/off toggle in the UI.
+3. Download format (PNG or JPEG).