### Summary

This PR improves GPUI's Criterion benchmark integration by adding
task-based measurement APIs to `BenchAppContext`. Benchmarks can now
await asynchronous GPUI operations whose completion depends on
foreground work, background work, timers, or external wakes, while
keeping setup and destruction outside the measured interval.

#### Added `BenchAppContext` APIs

- Add `bench_task` to measure a GPUI `Task` through completion in
Criterion's iteration loop.
- Add `bench_batched_task` to perform per-iteration setup outside the
timed interval before measuring a task.
- Preserve support for non-`Send` foreground task outputs and destroy
outputs after timing stops.
- Collect frame timings produced during asynchronous benchmark
iterations without including collection in the measurement.
- Drive task completion across temporary dispatcher quiescence and
externally delivered wakes.

Release Notes:

- N/A