Recording (Linux): write fragmented MP4 so a crash keeps the take #944

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.