`BenchReport`'s frame histograms only ever recorded a window's draw and
present timings, so a slow foreground task that never dirtied or drew a
window was invisible to `gpui::bench` consumers even though GPUI's
foreground journal already timed it. Delta hit this concretely: a
foreground task stalled for 76-130ms with no window draw in progress,
and the only way to see it in a benchmark was to manually queue a marker
task and time it by hand, outside the normal report.

GPUI already has the data needed to close this gap. The foreground
journal (`crate::profiler::journal`, merged separately in #62779) times
every foreground task poll, action handler, and input dispatch on the
main thread, independent of whether a window ever gets involved. This PR
wires that journal into `BenchAppContext`/`BenchReport` so the existing
benchmark harness reports it directly, instead of asking benchmark
authors to build their own timing side channel.

`TraceScope`, the type that already scopes frame-timing collection to a
measurement, now also owns a `ForegroundJournalCollector` created at the
same point it starts. Since a collector only observes journal entries
recorded after its own creation, per-iteration setup work (which runs
before the trace scope starts) is excluded from the measurement by
construction, the same way it already was for draw/present timings.
Drained task poll, action, and input events are recorded into a new
`foreground_work` duration histogram on `BenchReport` (draws and
presents are skipped there since the existing frame-timing histograms
already cover them), exposed through `BenchReport::foreground_work()` as
`count`/`total`/`max`/percentiles plus frame-budget overruns at the
report's configured FPS, and printed alongside the existing histograms.

Because this rides on the same per-task-poll instrumentation that
already powers GPUI's hang detection, it works for `bench_task`,
`bench_batched_task`, and `bench_renderer` without requiring a window at
all, and it aggregates every foreground event recorded during a
measurement (not just the first).

The `#[cfg(test)]`-only `install_test_foreground_journal` helper is
widened from `pub(super)` to `pub(crate)` so `bench_context`'s own tests
can install an isolated per-thread journal instead of sharing (and being
interfered by) whatever the shared production journal on that thread
happens to hold.

## Testing

- `cargo test -p gpui --features bench --lib bench_context` - two new
focused unit tests drive the foreground journal directly: one proves a
60ms synchronous task poll with no window is reported as foreground work
with no frame events recorded; the other proves an 80ms task poll before
the trace scope starts is excluded from the measured summary. A third
test exercises the real public API end to end, running
`BenchAppContext::bench_task` through an actual `criterion::Bencher`
(via `criterion::Criterion::bench_function`) and asserting the ~20ms
task shows up in `BenchReport::foreground_work()`.
- `cargo nextest run -p gpui --features bench --lib profiler` - the
existing foreground journal, hang detection, and frame-timing test
suites still pass unchanged.
- `cargo check -p benchmarks --benches` - the existing `#[gpui::bench]`
consumers (`bench_iter`, `bench_renderer`) still compile against the
updated `TraceScope`/`BenchReport` internals.
- `cargo fmt --package gpui -- --check` and `./script/clippy -p gpui
--features bench` are clean.

Release Notes:

- N/A