Bench: Std.Http.Server reference — reactor is 44x faster

[junjihashimoto]

Follow-up measurement on PR #3.

What this adds

• examples/BenchStdHttp/Main.lean — same three routes (`/health`, `/json`, `/echo`) but implemented on top of Lean 4.31's stock `Std.Http.Server` (Sofia Rodrigues, 2025). Compiled as `lean_exe bench_std_http`.
• New "Reference: `Std.Http.Server` (Lean 4.31 stock)" section in `docs/BENCHMARKS.md` with numbers side-by-side against the reactor and nginx.

Result

Same wrk load, same box (M-series, 16 core), same run:

server	/health RPS	/json RPS	p99 (ms)
`Std.Http.Server`	1 652	1 622	178–196
lean-tea reactor	73 533	69 069	2.1
nginx	72 183	66 384	2.0

Reactor is ~44 × the throughput of the stock Std server at ~1/80 the p99 latency.

Why

Two overlapping causes:

1. `Std.Async.TCP` per-syscall hop. Same reason our own libuv `LeanTea.Net.Server` topped out at 6 k — every recv/send round-trips through the async task scheduler. `Std.Http.Server` inherits this ceiling directly.
2. Extra per-connection bookkeeping. `Std.Http.Server` layers `Std.Semaphore` (connection limit) + `Std.Mutex Nat` (active-conn counter) + `Std.CancellationContext` (shutdown coord) + the full RFC-9112 `Std.Http.Protocol.H1` streaming codec on top. The 178 ms p99 is characteristic of semaphore/mutex contention under keep-alive load.

That said: `Std.Http.Server` is the correct implementation — HTTP/1.1 pipelining, `Expect: 100-continue`, chunked streaming request bodies, graceful shutdown across in-flight requests, proper cancellation. The reactor gives up all of that for the 44×. Both belong in the framework; pick by workload. The BENCHMARKS.md prose says this.

Test plan

• `lake build bench_std_http` clean.
• Sanity: `curl /health` returns `OK`, `curl /json` returns the payload, `curl -X POST -d hello /echo` echoes.
• wrk numbers reproduced above.
• CI `bench.yml` — this PR doesn't touch the workflow; `Std.Http.Server` isn't in the tracker (yet). If we want it long-term as a trend series, add it as a separate follow-up.