docs: explain Rust data plane in README

[kfallah]

Summary

• explain that the local OpenAI-compatible API uses the native Rust data plane by default when available
• document the embedded Python fallback and shared authorization and usage ledger
• show the explicit --engine rust and --engine python controls and link the benchmark report

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run ty check
• targeted release evidence tests: 2 passed
• full suite: 2265 passed, 2 skipped, 13 failures in existing terminal-rendering expectations under the current local environment; the two dirty-checkout release-evidence failures passed after the commit

[greptile-apps]

Greptile Summary

The PR updates the API documentation to describe the Rust data plane's performance characteristics while retaining the existing programmatic gateway example.

Confidence Score: 4/5

The PR should not merge until the remaining default-engine claim is qualified with the conditions under which automatic Rust selection actually occurs.

Automatic engine selection still falls back to Python when the native extension is absent or the gateway root is uninitialized, while the README continues to call Rust the default without those prerequisites.

Files Needing Attention: README.md

Important Files Changed

Filename	Overview
README.md	Updates the local gateway documentation with a concise Rust-versus-Python performance statement.

_{Reviews (2): Last reviewed commit: "docs: clarify Rust gateway performance" | Re-trigger Greptile}