bench: add flow control regression repro

[brentechols]

Adds a standalone benchmark for the single-connection flow-control shape that showed the large regression while evaluating #917.

Default shape:

• requests=300
• connections=1
• streams_per_connection=8
• response_size=1048576
• stream_window_size=65535
• connection_window_size=65535
• worker_threads=8
• nodelay=false

Run with:

cargo bench --bench flow_control_repro

The shape can be adjusted with H2_BENCH_* environment variables for request count, connection count, streams, body sizes, windows, worker threads, idle/spin delay, and TCP_NODELAY.

This uses only the repo's existing dependencies/dev-dependencies; no new external crate is required.

Local validation:

• cargo fmt --check
• cargo check --benches
• cargo bench --bench flow_control_repro
  • completed=300
  • requests_per_second=34.971
  • latency_p95_us=324883.900

[brentechols]

Updated run with the corrected checked-in benchmark shape.

The benchmark now keeps the in-flight response futures inside the connection task, matching the local repro harness. The earlier draft PR body result came from a spawned-response-task shape and should be ignored.

Slow configuration:

H2_BENCH_REQUESTS=300
H2_BENCH_CONNECTIONS=1
H2_BENCH_STREAMS=8
H2_BENCH_RESPONSE_SIZE=1048576
H2_BENCH_STREAM_WINDOW_SIZE=65535
H2_BENCH_CONNECTION_WINDOW_SIZE=65535
H2_BENCH_WORKER_THREADS=8
H2_BENCH_NODELAY=false
cargo bench --bench flow_control_repro

Results from this machine:

code	elapsed_ms	requests_per_second	latency_p50_us	latency_p95_us	latency_p99_us	bytes
master (21211d0) + this benchmark	613.642	488.884	9039.200	32150.500	61184.700	314572800
#917 (c970de5) + this benchmark (321be65)	4213.694	71.196	109290.500	172660.000	220014.200	314572800

That puts #917 at ~0.146x the master baseline for this shape, or about 6.9x slower.

Validation after updating the benchmark:

• cargo fmt --check
• cargo check --bench flow_control_repro
• cargo bench --bench flow_control_repro on both master+bench and Refactor big mutex to reduce contention #917+bench

No new external dependencies are required; this uses the existing repo dependency set.