fix: Address production audit gaps for v1.0.1

[BitcoinErrorLog]

Summary

This PR addresses all critical, high-priority, and medium-priority issues identified in the production readiness audit.

Changes

Security Improvements

Lock Poisoning Resilience

• Rate Limiter: All Mutex locks now use unwrap_or_else(|e| e.into_inner()) instead of .unwrap(), preventing panics if a thread panics while holding a lock (9 instances fixed)
• Session Manager: ThreadSafeSessionManager now handles lock poisoning gracefully (8 instances fixed)

Optional Timestamp Validation

• IdentityPayload: Added optional expires_at: Option<u64> field for defense-in-depth timestamp validation
  • When set, server validates that current time is before expiration before processing
  • Validation occurs BEFORE signature verification (fail-fast)
  • Backward compatible: None means no expiration check (existing behavior)

Bug Fixes

• Test Suite: Fixed examples/storage_queue.rs to compile without storage-queue feature
  • The example now prints a helpful message when the feature is not enabled
  • Resolves cargo test --all failure

Code Quality

• Fixed clippy warnings in example files:
  • examples/xx_pattern.rs: Fixed unused variables and needless borrows
  • examples/server_example.rs: Fixed needless borrows and inefficient .iter().cloned().collect()
  • examples/error_handling.rs: Fixed literal format strings

Testing

• ✅ cargo build --all-targets --all-features passes
• ✅ cargo test --all passes (96 tests)
• ✅ cargo clippy --all-targets --all-features clean
• ✅ cargo doc --no-deps compiles
• ✅ paykit-rs integration verified (builds and tests pass)

Breaking Changes

None - All changes maintain backward compatibility:

• expires_at field defaults to None via serde
• Lock poisoning recovery maintains existing behavior
• No public API signature changes

Files Changed

• src/rate_limiter.rs: Lock poisoning handling
• src/session_manager.rs: Lock poisoning handling
• src/identity_payload.rs: Add expires_at field
• src/server.rs: Add timestamp validation
• src/client.rs: Support expires_at in handshake
• examples/storage_queue.rs: Fix conditional compilation
• examples/*.rs: Fix clippy warnings
• tests/*.rs: Update for new expires_at field
• fuzz/fuzz_targets/fuzz_identity_payload.rs: Add expires_at fuzzing
• CHANGELOG.md: Document v1.0.1 changes
• PRODUCTION_AUDIT_REPORT.md: Updated audit status