Synthetic test clips

Generated with ffmpeg (see git history / commit for exact commands). Ground truth for validating marker capture, offset math, and lossless export.

sync_test_INSYNC.mp4 — 10s, 640x480 @30fps, black video with a 33ms white flash at t=5.000s; near-silent 1kHz tone that jumps to full volume for the same 33ms window at t=5.000s. Video and audio events are aligned (offset = 0ms).
sync_test_OUTOFSYNC_audio+350ms.mp4 — same content, but the audio track has been shifted +350ms later relative to video (via -itsoffset 0.350 on the audio input, stream-copied). The flash event stays at video t=5.000s; the beep event now appears at t=5.350s.

Ground truth for the app's marker/offset workflow:

Mark Video at the flash → ~5.000s
Mark Audio at the beep → ~5.350s
Expected computed delay_ms = +350 (audio needs to be delayed to match video)
Exporting sync_test_OUTOFSYNC_audio+350ms.mp4 with delay_ms = +350 should reproduce (bit-for-bit, modulo container overhead) sync_test_INSYNC.mp4's sync.

Frame at exactly t=5.000s is frame index 150 (30fps) — useful for checking frame-step precision lands exactly on it.