perf(cache): consolidate HotSnapshot atomic refcounts into single Arc<FrozenState> (#149)

[ZhuchkaTriplesix]

Summary

• Consolidates all immutable request-path state fields into a single FrozenState struct, held as snapshot: Arc<FrozenState> on AppState.
• Replaces 8 individual Arc::clone / Py::clone atomic refcount bumps per request in AppState::hot_snapshot() with a single Arc::clone(&self.snapshot).
• Updates AppState::rebuild_snapshot() on all route/middleware/CORS/DB mutations.
• Eliminates redundant per-request cloning of cors, security_headers, and middleware in src/dispatch.rs.

Closes #149