frame-track reaches the dashboard: builder + REST index + reducer ingest

[reynsu]

Parent

#74

What to build

Make a recorded run's full frame track available to the dashboard as ordered, timestamped data (no playback UI yet). Add a pure frame-track builder (per-frame JSONL lines → an ordered track with timestamps; tolerates pre-feature runs that only have per-step stills). Expose the frame-track index for a run over the runs REST surface. Have the App reducer ingest per-frame `frame` lines into a per-test frame track (append on live, hydrate on replay).

Acceptance criteria

• Pure builder turns (possibly out-of-order) per-frame lines into an ordered track with correct timings; a run with only per-step stills yields a degenerate track without throwing (unit test).
• Runs REST returns the frame-track index for a run.
• Reducer holds a per-test frame track; live appends, replay hydrates.
• Selecting a replayed run leaves the full ordered track in state (asserted), even though the Preview still shows a still for now.
• Pre-feature persisted runs still load.

Blocked by

#77