Play split recordings as one: discover and concatenate colocated chunks

Owner direction (2026-08-01, future feature): Insta360 cameras split long recordings into sequential chunk files (X4 Air included; a 30-minute flight is _001.._006 in one folder). Selecting any single chunk should discover its colocated siblings and play the whole recording as one continuous timeline.

Depends on the same capability #123 is establishing: resolving a picked file to its real directory and reading colocated files (lens mates today, sequential chunks here). The #123 fix is being shaped so sibling discovery is a general capture-set resolution rather than a hard-coded two-file pair; this issue builds chunk concatenation on top (demuxer-level continuity, PTS stitching across chunk boundaries, gyro/exposure record continuity across files - each chunk carries its own trailer).

Not scheduled; pre-work happens in #123.