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!

src/lib/guardrails/videoBridgeRuntime.ts

@@ -339,6 +339,15 @@ export function calculateSamplingDecision(
   }
 
   const frameCount = uniform.length;
+  if (frameCount === 1) {
+    return {
+      candidateCount: candidates.length,
+      ...(focusWindow ? { focusWindow } : {}),
+      policyEffective: "uniform",
+      policyRequested: "scene_aware",
+      timestamps: uniform,
+    };
+  }
   const selected =
     candidates.length <= frameCount
       ? [...candidates]