Preview shows the webcam's first frame when a clip is entered past the webcam's end from another clip

[EtienneLescot]

Found in the v2.0.0-rc.13 Windows e2e pass (logged in #988).

What happens

When the playhead enters a clip by its end, from another clip, at a time past the webcam file's last frame, the preview shows the screen's last frame correctly but the webcam's first frame instead of its last.

Repro (Windows, rc.13)

1. Record a take with the camera on. In the take used here: screen video 50.98 s, webcam 51.24 s, audio and clip 51.31 s (audio runs past both videos).
2. Media → drag the same recording onto the timeline again, so you have two clips of one take.
3. Scrub into clip 2 (e.g. 1:10), then click the ruler at the very end of clip 1 (≈ 51.2 s, i.e. between the webcam's last frame and the clip end).

Expected: webcam shows its last frame (ground truth from ffmpeg: an empty chair from 18.7 s to the end).
Actual: webcam shows frame 0 (the person sitting there at the start of the take). Reproduced 2/2.

Controls, both correct:

• Same target reached from inside clip 1 (e.g. 0:40 → end): right webcam frame.
• From clip 2 to 51.0 s (before the webcam's last frame): right webcam frame.

Cause

At a clip switch, open_and_seek_clip and seek_pair in crates/compositor/src/live.rs seek the screen with seek_to_or_last (227b9b8) but the webcam with seek_to, and on a null frame fall back to wdec.seek_to(0.0):

let mut wf = wdec.seek_to(webcam_seek_time(source_time_sec, webcam_offset_sec))?;
if wf.is_null() {
    wf = wdec.seek_to(0.0)?;
}

present_frame (the same-clip path) already uses seek_to_or_last for the webcam (83fbcec), which is why the in-clip control is right. Still present on main.

Likely fix: seek_to_or_last for the webcam in both places (keeping the 0.0 fallback only for a target before the first frame, if that case exists).

Impact

Low: paused scrub only, in a window of ~70 ms at the end of each clip (the gap between the webcam's last frame and the clip end). Playback is not affected.