fix(video): make one-frame scene sampling deterministic (#11344)

Merged via consolidated batch validation. Makes scene_aware Video Bridge sampling deterministic for a one-frame budget: falls back to the midpoint of the active full-video/focus window and reports policyEffective: uniform (a single scene candidate can't preserve both temporal ends). Adds opt-in real-FFmpeg fixture matrix (rapid edge cuts, one-frame budget, static/gradual scenes, sub-second clips, detector failure). Static gates green; own regression suite (videoBridgeSampler.test.ts, video-bridge-sampler-ffmpeg.test.ts) passed in the combined-batch run. Related to #9760. Thanks!

docs/security/GUARDRAILS.md

@@ -328,7 +328,10 @@ fixed FFmpeg pass over the already validated local stream, select bounded
 uniform midpoints on detector failure, timeout, malformed output, or an empty
 candidate set. Segment-aware mode allocates midpoint samples proportionally to
 the validated scene intervals. The hard 16-frame cap is
-applied after selection in every policy. A caller may optionally provide a
+applied after selection in every policy. When a scene-aware request has only a
+one-frame budget, it uses the uniform midpoint of the active full-video or focus
+window and reports `policyEffective: uniform`: a single selected scene frame
+cannot preserve both temporal ends. A caller may optionally provide a
 finite focus window (`start`/`end` seconds); bounds are clamped to the media
 duration, reversed or non-finite windows are rejected, and all sampling
 policies are performed only inside the normalized interval. The resulting