sync_logic.py: pure delay_ms computation with the documented sign
convention. Caught and fixed a sign error in my own test-fixture
ground-truth doc while writing the unit tests: the audio+350ms fixture
actually requires delay_ms=-350 (delay video), not +350, since the
audio is already the late one.
ffmpeg_export.py: ports ffdelay's two-branch command structure
(offset video input vs audio input depending on delay_ms sign) plus
its documented quirks (leading-zero float formatting, -ss/-t applied
to both inputs for clip mode, non-fatal stream-copy warnings surfaced
via stderr rather than swallowed).
Added an end-to-end integration test (scripts/test_export_roundtrip.py)
that independently re-detects event timing in the exported output via
ffmpeg scene-change/silence detection (not the same code path used to
build the export) and confirms both the positive- and negative-delay
branches bring drift down from ~350ms to under 1ms.
Confirms single-frame stepping is reliable: monotonic frame counter
across steps, and the synthetic clip's 1-frame flash event lands on
exactly one frame with clean neighbors. Also documents/works around
three environment quirks hit along the way: locally-built libmpv not
in ldconfig's cache, QApplication resetting LC_NUMERIC and breaking
libmpv's locale requirement, and the playback-restart event not firing
reliably after frame-step (switched to polling estimated_frame_number).
ThePendulum