fix: keep adjacent clips contiguous when an export changes the frame rate

[moneymaker-ux]

A project is edited at 30 fps, and an export rebuilds it at video.fps from the export settings. syncTiming rounds a clip's start, sourceIn and sourceOut to frames of that rate one by one, so the clip's end on the timeline (start - sourceIn + sourceOut, in frames) can land a frame before or after the next clip's start. The export then shows a blank frame at the cut, or the two clips overlap for a frame.

Repro, exported at 25 fps (times on the 30 fps grid, as the canvas writes them):

<video src={...} start={0} sourceIn={0.033333} sourceOut={1.133333} />
<video src={...} start={1.1} sourceIn={0} sourceOut={2} />

The first clip ends at frame 27 (0 - round(0.83) + round(28.33)), the second starts at frame 28 (round(27.5)): frame 27 is blank.

Change

• syncTiming: a sourceOut is rounded where the end it implies falls on the timeline, start + (sourceOut - sourceIn) / rate, the way an authored end already is.
• toFrames: settles a time to the runtime's time base (snapToMs), then to a thousandth of a frame, then rounds it to a frame. An end worked out from three decimals lands a hair either side of the same instant written as the next clip's start, and on a half frame that hair rounded the two apart even with the first change alone.

Independent of #66.

Verification

• Timing math, property check on random pairs of adjacent clips (the second starts at start + (sourceOut - sourceIn) / rate of the first): times on 24, 25, 30 and 60 fps grids and on microseconds, exported at 24, 25, 30, 50 and 60 fps. Mismatched cuts out of 800,000: 200,037 before, 0 after. Of 80,000 clips whose start and length are whole frames of the export rate, 41 changed, each by one frame toward its authored length (the old rounding missed it on a half-frame tie).
• Real project, v0.204.2 (4a652f5) + fix: keep scheduled audio when a clip ends during an export #66, with and without this change: 77 s, 19 back-to-back cuts edited on the 30 fps canvas, exported at 2160×3840. Blank frames found by the luma of every frame (ffmpeg signalstats):

Export	Without this change	With this change
25 fps	one blank frame at 51.64 s	none
30 fps	none	none

• npm run check: the same output before and after (fails in packages/assets/src/browser.ts with TS2339, showSaveFilePicker, as on main). After rebasing onto main (2593b92), tsc --noEmit in packages/reconciler reports only that error.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AybThRmRHP14dmvwixan1h

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