Recording (Linux): write fragmented MP4 so a crash keeps the take

[EtienneLescot]

Part of #920 (rank 22). Effort S, risk medium.

Problem

• The Linux helper writes a plain MP4 (pipewire-capture/src/encoder.rs, muxer): its only index is written at the end, so a crash, a kill or a power loss mid-take loses the whole recording.
• Windows and macOS already write fragmented MP4 with one-second fragments for exactly that reason (fix(recording): write fragmented MP4 so a killed helper keeps its file #338, a6795d2). Linux never tried it.

Proposal

• Write fragmented MP4 (movflags=frag_keyframe+empty_moov+default_base_moof, or frag_duration of one second) on the Linux muxer.

Verify

• A test that writes frames, drops the muxer without the trailer (a simulated kill) and reads the file back: it opens and holds the frames written before the last complete fragment.
• The editor still seeks in a fragmented file (the compositor opens it); measure a seek before and after.

[github-actions]

Fixed by #949 and merged into main.

This change is assigned to the Next Release release milestone and is not necessarily available in the latest downloadable desktop release yet. It is currently marked as status: pending release until a packaged release containing it is published.

[EtienneLescot]

Moved into v2.0.0 as part of the pre-release cut.