Export runs up to one frame per span longer than its timeline #1162

Summary

An export runs longer than its timeline: every span (a stretch of one clip at one speed, between trims, regions and clip junctions) rounds its own frame count up, so the error grows with the number of spans, up to one frame per span.

Found in the v2.1.0 RC QA: a two-clip MP4 export (30 fps) ran 2451 frames where the arithmetic gives 2444.9.

Cause

push_speed_segment (crates/compositor/src/regions.rs) gives each span ceil((duration − 1 ms) / speed × fps) frames, and walk_composited_timeline restarts its sampling at each span's start. outputFrameCount.ts mirrors the same arithmetic, so the progress total carries the same error.

Evidence

QA project: 2 clips, 6 trims, a 1.5x and a 4x region per clip, so 14 spans.

• Programme length: 2444.86 frames.
• Sum of per-span ceilings: 2452, which is the UI's total.
• The export holds 2451 packets. Its clip ends are clamped to the screen stream's real length, 48.765 s, against a declared 48.789 s (the audio track's). Per-span ceilings over the clamped windows also give exactly 2451.

Headless repro: CLI export with the installed RC, a 20 s synthetic source, 60 fps.

Not defects:

• Audio: it always lasts exactly frames ÷ fps.
• Video duration: duration reads one frame short (81.667 s against 81.700 s) because the muxer records the last frame's timestamp, so the two tracks are in sync.

Fix

• Put the whole timeline on one frame grid: frame k at programme time k / fps, with programme time carried across spans, trims and clips.
• Each span emits the grid frames inside it, and samples the source where the grid puts them.
• The total becomes ⌈(P − 1 ms) × fps⌉ for P programme seconds, within one frame of the arithmetic whatever the span count.
• Audio follows, because it is stretched to each span's frame count.

Not fixed

The progress total still uses the declared clip length, so it is one frame high when a source's video is shorter than its container. That limitation is documented in outputFrameCount.ts.