Four critical fixes:
- Remove overflow:hidden on tlRef so Timeline.init's scroll survives re-renders
- Don't call _renderClips() inside mousedown (was destroying event target mid-drag)
- Use refs for undo history to eliminate stale closure in onClipsChanged callback
- Change .tl-clip-area overflow:hidden to overflow:visible so pointer events reach clip edges
When duration_ms is known, dragging the right-trim handle past the end
of the source clip could push timeline_out_frames beyond what the source
material covers. Cap the delta so neither timeline_out_frames nor
source_out_frames can extend past the available source frames.
Also changed assetFrames fallback from origSrcOut (prevents any extension
when duration is unknown) to null, so the guard is simply skipped when
we don't have duration metadata.
