fix: add macOS Homebrew Opus fallback and fix shutdown dict iteration

- Add Homebrew library path fallback when ctypes.util.find_library fails
  on macOS (Apple Silicon + Intel paths, guarded by platform check)
- Fix RuntimeError in gateway stop() by iterating over dict copy
- Update Opus tests to verify find_library-first + conditional fallback

gateway/run.py

@@ -754,7 +754,7 @@ class GatewayRunner:
         logger.info("Stopping gateway...")
         self._running = False
         
-        for platform, adapter in self.adapters.items():
+        for platform, adapter in list(self.adapters.items()):
             try:
                 await adapter.disconnect()
                 logger.info("✓ %s disconnected", platform.value)