Add synthetic ground-truth test clips (in-sync + audio+350ms)

.gitignore

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+__pycache__/
+*.pyc
+.venv/

test_media/README.md

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+# 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.