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/assets/visible-mark-candidates/README.md

@@ -20,6 +20,13 @@ ship; `scripts/vendor_mark_calibrate.py` is the candidate-detector harness.
 | `notebooklm_alpha.png` | NotebookLM wordmark (bottom-right) | 12 corpus files | locate geometry not yet fitted; POS max 0.12. |
 | `dola_alpha.png` | DolaAI on images (the video mark is registered) | 12 corpus files | POS 0.11-0.21 vs NEG max 0.30. |
 | `mindvideo_alpha.png` | MindVideo.AI (top-right) | 11 corpus files | POS 0.29-0.32 vs NEG max 0.30 -- borderline overlap, not shippable. |
+| `higgsfield_alpha.png` | HIGGSFIELD AI wordmark (bottom-right; the boxed `AI` variant shares the cohort) | 5 wordmark files (16 in the boxed-AI OCR cluster) | POS max 0.26 vs NEG max 0.22 -- no separation; the mark may be two-part (wordmark + boxed AI) and needs a composed template. |
+| `jianying_alpha.png` | 剪映AI (CapCut's CN sibling, bottom-right) | 2 corpus files | POS 0.29 vs NEG max 0.35. |
+| `capcut_alpha.png` | CapCut AI pill (top-left; likely pill class, not plain text) | 3 corpus files | POS max 0.15 vs NEG max 0.33 -- locate geometry not yet fitted for the pill form. |
+| `zsky_alpha.png` | MADE WITH zsky.ai (bottom-right) | 2 corpus files | POS 0.10 vs NEG max 0.27. |
+| `chromastudio_alpha.png` | ChromaStudio.ai (bottom-right) | 2 corpus files | POS 0.11 vs NEG max 0.29. |
+| `digenai_alpha.png` | DIGENAI (bottom-right) | 3 corpus files (one 2026-07-24 batch) | POS 0.16 vs NEG max 0.31. |
+| `gendo_alpha.png` | GendoAI (bottom-left) | 3 corpus files | POS 0.08 vs NEG max 0.32. |
 | `xinghui_alpha.png` | 星绘AI生成 (parked before this set) | -- | prior parking, unchanged. |
 | `qingyan_alpha.png` | 清言·AI生成 (parked before this set) | -- | prior parking, unchanged. |
 | `hailuo_alpha.png` | Hailuo AI image wordmark (parked before this set; the VIDEO label is registered) | -- | prior parking, unchanged. |