Add synthetic visible-mark example gallery with canary tests

One committed example per registered mark: 12 PNG (image registry) and 6 MP4
clips (video registry), generated by scripts/render_visible_examples.py from
the committed silhouettes and detector templates -- never from user uploads.
The generator self-verifies (exit 1 when a mark misses its own example) and
tests/test_visible_examples.py holds both sides to it: registry completeness
both ways, per-engine detection on the canonical example, and the shipped
temporal selection accepting each clip.

Second tranche of measured-but-unregistered candidates parked under
scripts/assets/visible-mark-candidates/ with a README recording why none
ships yet (positives do not separate from clean negatives): samsung_en,
gemini_text, notebooklm, dola, mindvideo, higgsfield, jianying, capcut, zsky,
chromastudio, digenai, gendo.

tests/test_visible_examples.py

@@ -0,0 +1,57 @@
+"""The visible-mark example gallery is complete and self-consistent.
+
+Two failures this suite exists to catch:
+  * a mark registered without a committed example (the gallery lags the registry);
+  * an engine that no longer detects its own canonical example (the gallery is
+    generated from the engines' measured geometry, so this is a regression tripwire).
+
+The examples are SYNTHETIC (``scripts/render_visible_examples.py`` composites the
+committed silhouettes onto a generated base). User uploads never enter the repo.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from remove_ai_watermarks import watermark_registry as wr
+from remove_ai_watermarks.image_io import imread
+from remove_ai_watermarks.video import VIDEO_VISIBLE_MARKS, identify_video
+
+_ROOT = Path(__file__).resolve().parents[1]
+_GALLERY = _ROOT / "data" / "fixtures" / "visible"
+
+_IMAGE_KEYS = [m.key for m in wr.known_marks()]
+
+
+class TestGallery:
+    def test_every_registered_mark_has_an_example(self) -> None:
+        missing = [key for key in _IMAGE_KEYS if not (_GALLERY / key / "example.png").is_file()]
+        assert missing == [], f"registered without an example: {missing}; run scripts/render_visible_examples.py"
+
+    @pytest.mark.parametrize("key", _IMAGE_KEYS)
+    def test_engine_detects_its_own_example(self, key: str) -> None:
+        img = imread(str(_GALLERY / key / "example.png"))
+        assert img is not None, key
+        det = wr.get_mark(key).detect(img, provenance=False)
+        assert det.detected, f"{key}: confidence {det.confidence:.3f} on its own example"
+
+    def test_gallery_has_no_stray_directories(self) -> None:
+        known = set(_IMAGE_KEYS) | set(VIDEO_VISIBLE_MARKS) | {"README.md"}
+        extra = sorted(p.name for p in _GALLERY.iterdir() if p.name not in known)
+        assert extra == [], f"gallery holds unregistered examples: {extra}; remove or register them"
+
+
+class TestVideoGallery:
+    def test_every_registered_video_mark_has_an_example(self) -> None:
+        missing = [key for key in VIDEO_VISIBLE_MARKS if not (_GALLERY / key / "example.mp4").is_file()]
+        assert missing == [], f"video mark without an example: {missing}; run scripts/render_visible_examples.py"
+
+    def test_selection_accepts_each_example(self) -> None:
+        # The shipped temporal selection (not just the per-frame detector) must
+        # accept the clip: table order resolves cross-template ties, so the example
+        # must carry the discriminative variant of its mark.
+        for key in VIDEO_VISIBLE_MARKS:
+            rep = identify_video(_GALLERY / key / "example.mp4", check_visible=True)
+            assert rep.visible_mark == key, f"{key}: selection returned {rep.visible_mark!r}"