Prevent KeyEventMonitor launch crash

[danman231]

Summary

• prevent Hex from synchronously re-entering its KeyEventMonitor queue when Input Monitoring is denied or reported stale
• protect the event-proven Input Monitoring flag with a dedicated lock
• add deterministic denied-permission regression coverage and a patch changeset

Root cause

handleInputEvent registers its first handler inside a barrier block on com.kitlangton.Hex.KeyEventMonitor and calls startMonitoring. When IOHIDCheckAccess reports denied, currentInputMonitoringTrust() synchronously dispatched back onto that same queue. Libdispatch terminates the app with SIGTRAP instead of allowing the self-sync.

The permission-proof flag now uses its own lock, so callers can safely read it from either inside or outside the state queue while preserving the stale-TCC recovery introduced for #250.

Validation

• regression test reproduced the original BUG IN CLIENT OF LIBDISPATCH: dispatch_sync called on queue already owned by current thread failure before the fix
• full Xcode test suite passes, including KeyEventMonitorClientTests
• HexCore test suite passes: 67 tests
• optimized Release build succeeds
• fixed Release app remained alive through a 15-second launch smoke test and generated no crash report

Fixes #254

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