fix(timeline): quantise the scene timeline once instead of per scene

[LeadcodeDev]

Closes #372 (the half that #391 left open).

The defect

Two 1.05 s scenes rendered 64 frames where 2.1 s at 30 fps is 63: each scene
rounded 31.5 up on its own and the error accumulated. Twenty 0.35 s beats ended
10 frames past their soundtrack.

The model

The timeline is defined in seconds first and quantised once. quantised_spans
walks the scene durations and the transition overlaps as real numbers, then rounds
each cumulative boundary. A scene's frame count is the difference of two rounded
boundaries, so the half frame is paid once by whichever side the boundary rounds
toward — never twice.

A transition's overlap falls out of the same arithmetic: it is
spans[i].end - spans[i+1].start, not a separately rounded figure that could
disagree with the scenes around it.

Four sites, and they did not agree

Site	Was	Now
encode/video/tasks.rs (v1)	round(d·fps) per scene, minus separately rounded transitions	derives from the spans
encode/video/tasks.rs (v2)	cursor accumulated rounded frames	cursor carries seconds, so at resolves on the same grid as automatic placement
encode/video_audio.rs	rounded each scene again to place audio	reads the spans
cli/commands/migrate.rs	computed the v2 at/duration from its own per-scene rounding	derives both from quantised_spans
cli/commands/info.rs	printed per-scene counts that summed past its own total (32 + 32 against 63)	prints the spans

video_audio.rs mattering here is not cosmetic: it places an embedded video's
audio slice, so it was drifting from that video's own picture by a frame per scene.

migrate caught itself

migrate's own frame-identity check is what surfaced the fourth site — migration
of the six-scene fixture went 405 → 404 as soon as the render side was corrected.
It now derives from the same function, and the check passes:

Duration: 13.500s -> 13.500s (405 -> 405 frames @ 30fps) — frame-identical

One expected value changed, and why

snap_beat_on_top_of_the_migrated_file_moves_cuts_and_changes_duration asserted
15.0 s. That number was an artifact of the old chain: scene 4 was given 51 frames
where its window at the snapped start is 52 (round(379.27) - round(327.27)).

Recomputed by hand from the migrated file's own at values (0, 1.9, 4.2, 6.5, 9.3667, 11.1) against the bpm-11 beat grid (0 s, 5.4545 s, 10.9091 s), with the
three clamped-forward cuts, the answer is 451 frames. The test now pins that,
and states the structure it comes from rather than a bare literal.

The issue's repros, end to end

	info	rendered (ffprobe)
two 1.05 s scenes	2.1 s, 63 frames	63
v2 at: 0 / at: 1.0, both 2.0 s	3.0 s, 90 frames	90
two 1.0 s scenes, 0.6 s iris	1.4 s, 42 frames	42

Reverts that bite

Restoring the per-scene round(d·fps) inside quantised_spans fails three of the
six new tests:

2.1s at 30fps is 63 frames; rounding each 1.05s scene on its own gives 32 + 32
20 beats of 0.35s is 7.0s exactly; rounding 10.5 up twenty times ends 10 frames late
the half frame is paid once, by whichever scene the boundary rounds toward — it is never paid twice

Gate

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean · cargo test --workspace 1777 passed.