test(inspector): no coverage for RunsRootWatcher poll / idle-on-finished / warming-up state machine

[jack-champagne]

Surfaced in review of #23.

The SinkDedup primitive is now unit-tested, but RunsRootWatcher's new state machine has zero automated coverage and is the most intricate code in the β release — validated only by manual runs:

• the 700ms tick() poll backstop (catches frames/FINISHED that fs.watch/FSEvents drops),
• the idle-on-finished baseline at start() (a finished run at launch sets activeRunDir without a sink so it isn't re-rendered),
• the warming-up → first-frame transition.

Fix: an integration test that appends iter_*.png + AMICODE_ITER lines to a temp run dir over time and asserts the sink/inspector calls — including a finished run at launch staying idle, warming → first frame, and the poll delivering frames fs.watch missed.

Ref: #23.

[jack-champagne]

Resolved by #23 (merged, 9a80c67). test/watcher_statemachine.test.ts drives the real RunsRootWatcher over a temp run dir via tick() and asserts the three states this issue named: finished-at-launch stays idle (no stale frame), fresh run → warming-up, poll backstop delivers the newest frame (newest-wins on coalesced ticks), then terminal completion. Pure fs + mocked inspector — runs in CI, no binary dependency.