Protect the global `instance` variable with sync.RWMutex to prevent
data races when SetInstance/App() are called concurrently (especially
in tests).
Changes:
- Add instanceMu mutex to protect instance variable
- Update App() to use RLock for reading
- Update SetInstance() to use Lock for writing
- Add GetInstance() for non-panicking access
- Add ClearInstance() for test cleanup
- Update tests to use new thread-safe functions
- Add concurrent access test with race detector
Closes#84
Co-authored-by: Claude <noreply@anthropic.com>
04e70d9cda