feat(visible): Samsung Galaxy AI mark removal (bottom-left reverse-alpha, #37)

New samsung_engine.py mirrors the jimeng engine but anchors bottom-left; wired
into watermark_registry, the CLI (--mark samsung / auto), and identify
(visible_samsung, medium). visible_alpha_solve.py gains a corner=bl mode;
samsung_alpha.png solved from @f-liva's flat captures. Calibrated for the
Italian "Contenuti generati dall'AI" variant. Flat black/gray/white captures
committed, real photos gitignored. Tests + docs.

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

data/samsung_capture/README.md

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+# Samsung Galaxy AI visible watermark capture
+
+> **Status (built 2026-06-05):** flat black/gray/white Samsung Galaxy AI captures
+> were obtained (issue #37, from @f-liva) and the alpha map was solved. Removal is
+> reverse-alpha plus a thin residual inpaint over the glyph footprint; see the
+> `samsung_engine.py` notes in the root `CLAUDE.md`. The text below is the capture
+> plan and the open quality follow-up.
+
+Goal: capture the Samsung Galaxy AI "✦ Contenuti generati dall'AI" visible wordmark
+over known flat backgrounds so we can build a per-pixel alpha map and a reverse-alpha
+remover, the same way the Gemini sparkle and the Doubao / Jimeng strips work
+(`src/remove_ai_watermarks/gemini_engine.py`, `doubao_engine.py`, `jimeng_engine.py`).
+
+## What we learned (verified from the captures, 2026-06-05)
+
+- Mark: a sparkle icon followed by the locale string "Contenuti generati dall'AI"
+  (Italian), a light low-opacity (peak alpha ~0.38) semi-transparent **white**
+  overlay, anchored **bottom-LEFT** (Doubao/Jimeng are bottom-right). The string is
+  locale-specific, so the alpha template is per-locale; this build is the Italian
+  variant. Other locales need their own captured template.
+- Blend model: alpha compositing with a pure-white logo, `watermarked =
+  a*255 + (1-a)*original`, solved from the GRAY capture (same careful recipe as
+  Doubao/Jimeng: cubic-background fit, mean over channels, full halo extent,
+  unblurred). The white capture confirms the logo is white; on white the mark is
+  white-on-white and not detectable (no contrast), which is fine -- there is nothing
+  to recover there.
+- Geometry (fraction of image WIDTH): asset width ~0.32, height ~0.038, left margin
+  ~0.011, bottom margin ~0.006. The mark scales with width: a 1086-wide flat capture
+  and a 2958-wide real photo both measure width_frac ~0.31.
+- **Resolution caveat (open quality follow-up):** the flat black/gray/white captures
+  arrived at the phone's flat-edit size (1086 wide and a landscape 1920 set), while
+  the real photos are ~3000 wide, so the captured glyph (~334 px) is ~2.7x smaller
+  than on a real photo (~900 px). The alpha is solved at the capture size and
+  width-scaled + NCC-aligned per image, which removes the mark cleanly (verified on a
+  real 2958-wide photo: re-detect 0.79 -> 0.00, no readable text or outline), but a
+  flat capture taken at the real photo resolution (~3000 wide) would let the alpha be
+  pixel-sharp instead of upscaled. Not a blocker; a quality upgrade if a full-res
+  flat capture is provided.
+
+## Capture protocol (to re-capture or add a locale)
+
+On a Samsung Galaxy AI device (set the UI language to the target locale):
+
+1. Run the AI edit (Generative Edit / Sketch to Image) on a solid black image, so
+   the overlay lands on a flat black background. Download the ORIGINAL output file
+   (not a screenshot, no crop or re-save).
+2. Repeat over solid white and solid gray (those pin the exact glyph color).
+3. Ideally run all three flat edits at the same resolution as real photos (~3000
+   wide) so the alpha map is pixel-sharp rather than upscaled.
+4. Plus 3-5 real outputs with the visible mark over normal content for validation.
+
+## Files
+
+- `captures/samsung_black_1.png`, `samsung_gray_1.png`, `samsung_white_1.png` --
+  portrait flat edits (1086 wide), the primary calibration set.
+- `captures/samsung_black_2.png`, `samsung_gray_2.png`, `samsung_white_2.png` --
+  a second (landscape 1920) set.
+- `captures/samsung_content_*` -- real-photo validation downloads, **gitignored**
+  (user content, repo is public).
+- `seeds/` -- synthetic solid-color inputs, gitignored (regenerable).
+
+Rebuild the alpha asset with:
+
+```
+uv run python scripts/visible_alpha_solve.py samsung
+```