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>

tests/test_cli.py

@@ -27,7 +27,9 @@ def runner():
 @pytest.fixture
 def sample_png(tmp_path: Path) -> Path:
     """Create a sample PNG for CLI testing."""
-    img = np.random.randint(0, 255, (200, 200, 3), dtype=np.uint8)
+    # Seeded: an unseeded random corner can occasionally trip the Doubao
+    # visible-mark detector, making `visible --mark auto` flaky.
+    img = np.random.default_rng(0).integers(0, 255, (200, 200, 3), dtype=np.uint8)
     path = tmp_path / "input.png"
     cv2.imwrite(str(path), img)
     return path
@@ -37,8 +39,9 @@ def _make_batch_dir(tmp_path: Path, count: int = 3) -> Path:
     """Create a directory with test images for batch testing."""
     input_dir = tmp_path / "input"
     input_dir.mkdir()
+    rng = np.random.default_rng(0)
     for i in range(count):
-        img = np.random.randint(0, 255, (100, 100, 3), dtype=np.uint8)
+        img = rng.integers(0, 255, (100, 100, 3), dtype=np.uint8)
         cv2.imwrite(str(input_dir / f"img_{i}.png"), img)
     return input_dir
 
@@ -119,7 +122,8 @@ class TestVisibleCommand:
     def test_visible_help(self, runner):
         result = runner.invoke(main, ["visible", "--help"])
         assert result.exit_code == 0
-        assert "Gemini watermark" in result.output
+        assert "visible AI watermark" in result.output
+        assert "--mark" in result.output
 
     def test_visible_basic(self, runner, sample_png, tmp_path):
         output = tmp_path / "clean.png"