frame-track-retention: keep recordings for the most recent 5 runs

[reynsu]

Parent

#74

What to build

Bound the disk cost of recordings. Add a pure `frame-track-retention` module: given the list of runs, it decides which runs keep their full frame track (the most recent 5) and which degrade to the last-frame-per-step they already have. Wire it into the run lifecycle so old frame tracks are pruned, while the degraded runs still render their per-step stills (and still appear in the RunPicker).

Acceptance criteria

• Pure module: given N runs, returns exactly the most-recent-5 to keep and the rest to degrade (deterministic unit test).
• After 6+ runs, the oldest run's per-frame sequence is pruned from `.reactlens/runs/`, but a representative still per step remains.
• A degraded run still loads and shows per-step stills (no playback); it still lists in the RunPicker.
• Pruning never touches the most recent 5 runs' tracks.

Blocked by

#77