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.

scripts/render_vendor_silhouettes.py

@@ -69,6 +69,15 @@ MARKS = {
     "notebooklm_alpha.png": "NotebookLM",
     "dola_alpha.png": "DolaAI",
     "mindvideo_alpha.png": "MindVideo.AI",
+    "higgsfield_alpha.png": "HIGGSFIELD AI",
+    "capcut_alpha.png": "CapCut AI",
+    "zsky_alpha.png": "MADE WITH zsky.ai",
+    "chromastudio_alpha.png": "ChromaStudio.ai",
+    "digenai_alpha.png": "DIGENAI",
+    "gendo_alpha.png": "GendoAI",
+    # CapCut's Chinese sibling, JianYing, stamps 剪映AI bottom-right (the
+    # international CapCut pill sits top-left).
+    "jianying_alpha.png": "剪映AI",
 }
 _REGISTERED = {f"{key}_alpha.png" for key in mark_keys()} & MARKS.keys()