feat(review): GT-aware TP/FP/FN frame annotation + scene-detector examples

dump_error_frames.py drew every identified box green, so a false positive looked
like a true positive and a missed cast member was invisible. Make the annotation
ground-truth aware, matching what the per-second scorer classifies:
  - GREEN  true positive  — a name X-Ray also credits to this scene
  - RED    false positive — a name X-Ray does NOT credit here (the real error)
  - ORANGE unknown detection
  - BLUE   a text panel listing X-Ray cast present with no detected face (the
           structural false-negatives — no box exists to draw)

Add two representative annotated frames to the scene-detector page: a clean
green-TP second, and the face-vs-scene-cast case (a red FP lead + six off-camera
cast in blue) that makes the recall ceiling visual. Frames are generated by the
script from replay.py --raw-out output; the two committed examples are hand-picked
doc assets (bulk experiments/dump_review is regenerable and gitignored).

docs/scene-boundary-detector.md

@@ -133,6 +133,25 @@ The two headline cases:
 Naive flood-fill barely beat doing nothing (64% vs 62%) and broke a film. With a
 real boundary detector, flood-fill is decisively the right mode.
 
+### What the frames look like
+
+`scripts/optimizer/dump_error_frames.py` pulls representative seconds and draws
+each face box coloured against X-Ray's scene cast: **green** = true positive (a
+name X-Ray also credits to this scene), **red** = false positive (a name X-Ray
+does *not* credit here — the real error), **orange** = an unknown detection. Cast
+X-Ray lists as present but for whom no face was detected — the structural
+false-negatives a face pipeline can never box — are listed as a **blue** panel.
+
+![A correctly identified second: green true-positive boxes](assets/images/scarface_tp_example.jpg)
+
+Above: three faces named correctly (green). Below: the face-vs-scene-cast tension
+made visual — the one visible face is confidently named (here it is a red
+false-positive, a lead X-Ray did not credit to this exact scene), while six
+credited cast members are off-camera with no face to detect (blue). This is why
+recall against X-Ray has a structural ceiling, not a fixable bug.
+
+![A false-positive box (red) with off-screen cast listed (blue)](assets/images/scarface_fn_fp_example.jpg)
+
 ## In the pipeline
 
 Boundary detection is a **post-EOF step**, like flood-fill itself: the per-film