runtime.LockOSThread/UnlockOSThread added to withEndpointVolume (volume)
and Capturer.run (viz). COM apartments are thread-affine on Windows;
without the lock the Go scheduler can migrate a goroutine mid-call to a
thread that never called CoInitializeEx, causing sporadic failures.

Also check CoInitializeEx HRESULT: S_OK (0) and S_FALSE (1) are success,
anything else is a real error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>