frame-track: persist the screencast as a per-frame sequence (no overwrite)

[reynsu]

Parent

#74

What to build

Stop collapsing the screencast to one frame per step. Persist every frame of a test as a monotonic per-test sequence on disk (e.g. `frames//.jpg`) plus a per-frame JSONL index line carrying `ts`, `stepId`, and the frame ref. Extract a deep, testable `frame-track` module that owns the sequence numbering, the path, and the index line; the event-persistor delegates to it. Add the per-frame path to the runs layout owner (`run-paths`).

Acceptance criteria

• A run writes a sequence of jpgs per test (one per received screencast frame), not a single overwritten file per step.
• `events.jsonl` gains one per-frame line per frame, each with `ts` + `stepId` + frame ref.
• `frame-track` module is unit-tested in isolation (sequence numbering, index line shape).
• Integration test: driving the persistor with a frame stream lands the per-frame sequence + index on disk without overwriting (prior art: tests/integration/replay-from-disk.test.ts).
• DISK-shape vs WIRE-shape frame boundary is preserved (the persistor only writes; it doesn't re-ingest disk frames).

Blocked by

#76