fix: bound stateful upstream request window

[mjc]

Summary

• keep up to 16 validated stateful requests in one upstream write window
• flush before backend reads and at the bound while preserving response ordering
• add a proof test where the backend waits for two commands before replying
• add matched stateful and per-command proxy benchmarks

Evidence

• nextest: 2278/2278 passed
• clippy with all features and warnings denied passed
• stateful pair median: 55.42 us
• per-command pair median: 66.27 us
• stateful pair: 2.364 GB/s; per-command pair: 1.977 GB/s

All commits are GPG-signed.

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📥 Commits

Reviewing files that changed from the base of the PR and between 03ea015 and 09ee30e.

📒 Files selected for processing (2)

• benches/end_to_end_proxy.rs
• src/session/handlers/stateful.rs

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