#3921 stagecraft remote studio poll: follow-ups from #3658 review (flaky test, mid-poll first page, quiet-poll test)

Low-severity findings from the verify-reviews run on swamp-club #3658 (PR 567), in stagecraft/extensions/models/_lib/engine/.

1. Flaky stop-on-leave test (flagged by both reviews). studio_serve_test.ts: after the event stream is cancelled, the test pauses a fixed 100ms before asserting no reads for 300ms with a 20ms poll interval. On a loaded runner the server may unsubscribe later and a poll lands in the window. Fix: wait with until() for the call count to stay flat across several poll intervals, or expose a listener count to wait on.

2. First page arriving during an in-flight poll waits double. studio_serve.ts serveRemote: subscribe() resets the delay to base on the first listener, but if a poll is in flight its finally computes nextPollDelay(base, unchanged) = 2x base, so the new page's next read is 30s rather than 15s. Fix: a reset flag set in subscribe that finally honours.

3. Backoff relies on an unchanged poll emitting nothing. Any emit from factories or work items marks the poll as changed. Add a test that an unchanged poll emits no event (remoteFactories.poll and watchWorkItems.tick), so the backoff cannot silently stop working.