Dropped entities are released only inside an update's effect flush, and
releases cascade: one flush drops the entities whose handles are gone,
their drops release further handles and can queue foreground work, and a
later flush collects those. `BenchAppContext` callers that only pump the
executor between iterations therefore saw torn-down state linger in the
entity map until some woken task happened to run an update — in a
downstream benchmark this looked like a per-iteration leak of the whole
app graph (~35 MB per iteration), releasing on an apparently timer-bound
schedule.

This adds `BenchAppContext::settle`, which alternates draining queued
work with GPUI update cycles until the dispatcher reports idle,
mirroring the update cadence production gets for free from frames and
input events. `bench_batched_task` now settles before each iteration's
setup (outside the timed interval), so the previous iteration's state is
fully released and cannot accumulate across a measurement. A new
`ThreadedDispatcher::is_idle` predicate backs the loop's termination and
is covered by a unit test.

Release Notes:

- N/A