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.

data/README.md

@@ -7,6 +7,8 @@ data/
   fixtures/
     provenance/   Real format and provenance fixtures used by tests
                   (source records live in fixtures/README.md)
+    visible/      Synthetic per-mark example gallery (one committed example per
+                  registered visible mark; see fixtures/visible/README.md)
   calibration/
     <vendor>/     Minimal controlled inputs needed to rebuild detector assets
   synthid/