perf: parking_lot mutex for route/OpenAPI state (issue #4)

[ZhuchkaTriplesix]

Summary

Replaces std::sync::Mutex with parking_lot::Mutex for AppState and all per-method matchit routers plus the OpenAPI value lock. parking_lot uses a smaller, faster lock implementation and has no poison semantics, so the old lock_err path is removed.

Note

Issue #4 also mentions compiled route snapshots / ArcSwap after freeze(); this PR is the low-risk mutex swap only. Further work can build on the same parking_lot types.

Tests: cargo test, full pytest (30).

Closes #4

[ZhuchkaTriplesix]

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