fix: pin COM/WASAPI goroutines to OS thread

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>

internal/viz/capture.go

@@ -10,6 +10,7 @@ import (
 	"log"
 	"math"
 	"math/cmplx"
+	"runtime"
 	"syscall"
 	"time"
 	"unsafe"
@@ -137,7 +138,15 @@ func (c *Capturer) Start(ctx context.Context) {
 }
 
 func (c *Capturer) run(ctx context.Context) error {
-	coInitializeEx.Call(0, 0) // COINIT_MULTITHREADED
+	// Pin this goroutine to its OS thread — WASAPI COM objects are
+	// thread-affine; the scheduler must not migrate us mid-call.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	hr, _, _ := coInitializeEx.Call(0, 0) // COINIT_MULTITHREADED
+	if hr > 1 {                            // S_OK=0, S_FALSE=1 are both success
+		return fmt.Errorf("CoInitializeEx: HRESULT 0x%08X", hr)
+	}
 	defer coUninitialize.Call()
 
 	// ── IMMDeviceEnumerator ──────────────────────────────────────────────────

internal/volume/volume.go

@@ -7,6 +7,7 @@ package volume
 
 import (
 	"fmt"
+	"runtime"
 	"syscall"
 	"unsafe"
 
@@ -91,13 +92,26 @@ var (
 
 // withEndpointVolume acquires an IAudioEndpointVolume for the default render
 // device, calls fn with its pointer, and releases all COM objects.
+//
+// COM apartments are thread-affine on Windows. LockOSThread pins this
+// goroutine to its current OS thread for the duration of the call so that
+// every COM call runs on the thread that initialized COM.
 func withEndpointVolume(fn func(vol uintptr) error) error {
-	coInitializeEx.Call(0, 0) // COINIT_MULTITHREADED
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	// CoInitializeEx returns S_OK (0) on first init, S_FALSE (1) if already
+	// initialized on this thread. Both are success. Anything ≥ 2 is an error
+	// (HRESULT error codes are 0x8xxxxxxx, always > 1 as unsigned).
+	hr, _, _ := coInitializeEx.Call(0, 0) // COINIT_MULTITHREADED
+	if hr > 1 {
+		return fmt.Errorf("CoInitializeEx: HRESULT 0x%08X", hr)
+	}
 	defer coUninitialize.Call()
 
 	// CoCreateInstance(CLSID_MMDeviceEnumerator) → IMMDeviceEnumerator
 	var enumerator uintptr
-	hr, _, _ := coCreateInstance.Call(
+	hr, _, _ = coCreateInstance.Call(
 		uintptr(unsafe.Pointer(&clsidMMDeviceEnumerator)),
 		0,
 		clsctxAll,