## Objective

Make renderer benchmarks exercise GPUI's scheduled frame callback
instead of drawing synchronously whenever application effects are
flushed. Add finite rendering sessions for workloads that need to run
until explicit completion, with the goal of exposing expensive
foreground work and rendering stalls without artificial display waits.

## Changes

- Keep automatic drawing during effect flushing test-only. Benchmark
updates schedule frames, and the benchmark context scans existing
per-window scheduling state to deliver callbacks outside the App borrow.
- Deliver pending frames after foreground task polls and benchmark
input/workload callbacks. Nested actions and effects complete before
frame delivery.
- Add `bench_renderer_session` with fresh untimed setup, an input-turn
index, a stop flag, a deadline, and final presentation before successful
completion. Session-owned state is dropped outside measurement so it can
cancel unfinished work.
- Present setup's initial frame batch before tracing starts and activate
benchmark windows to avoid unintended inactive-window animation
throttling.
- Stop task benchmarks as soon as their result is ready, before
delivering an unrelated extra animation frame. Renderer sessions
separately retain their final-presentation guarantee.

## Scheduling and scope

This is an **unpaced work-and-render benchmark**, not a simulation of
display latency or an OS event loop. Renderer input turns service up to
the initial ready-task count, delivering frames between polls; rendering
after a poll does not inject another input. The report's FPS affects
reporting only, not execution. Individual task polls and frame callbacks
cannot be preempted, so an external process timeout is still needed for
work that never returns.

This is the second part of the benchmark split following #64099, which
has merged. Expanded timing/attribution reporting, intermediate-texture
reuse, and native event-loop integration are deferred.

## Validation

Passed after merging current `main`:

- `corgi fmt -p gpui -- --check`
- `corgi check -p gpui --features bench-support --lib`
- `corgi check -p gpui --lib`
- `git diff --check origin/main...HEAD`

This PR intentionally contains no added tests while the implementation
is being reviewed. Existing test modules are unchanged. Runtime tests
and downstream Delta smoke runs have not been performed for this
redesigned scheduling policy; checks from the earlier implementation are
not validation of this version.

Release Notes:

- N/A