fix: add stub implementation for CGO audio on non-ARM platforms

Provides no-op AudioSource implementations for platforms that don't
support ARM CGO audio (x86_64, darwin, etc.). This allows golangci-lint
to run successfully on any platform without requiring ARM cross-compilation
toolchain.

The stub implementations return errors when called, ensuring that if
they're accidentally used at runtime on non-ARM platforms, it will fail
gracefully with a clear error message rather than undefined symbols.

Build constraints ensure the real CGO implementation is used on linux/arm
and linux/arm64, while stubs are used everywhere else.

internal/audio/cgo_source_stub.go

@@ -0,0 +1,50 @@
+//go:build !linux || (!arm && !arm64)
+
+package audio
+
+import "fmt"
+
+// StubSource is a no-op implementation for platforms without CGO audio support
+type StubSource struct {
+	direction  string
+	alsaDevice string
+}
+
+// NewCgoOutputSource creates a stub audio source for output (unsupported platforms)
+func NewCgoOutputSource(alsaDevice string) AudioSource {
+	return &StubSource{
+		direction:  "output",
+		alsaDevice: alsaDevice,
+	}
+}
+
+// NewCgoInputSource creates a stub audio source for input (unsupported platforms)
+func NewCgoInputSource(alsaDevice string) AudioSource {
+	return &StubSource{
+		direction:  "input",
+		alsaDevice: alsaDevice,
+	}
+}
+
+// Connect returns an error indicating CGO audio is not supported
+func (s *StubSource) Connect() error {
+	return fmt.Errorf("CGO audio not supported on this platform")
+}
+
+// Disconnect is a no-op
+func (s *StubSource) Disconnect() {}
+
+// IsConnected returns false
+func (s *StubSource) IsConnected() bool {
+	return false
+}
+
+// ReadMessage returns an error
+func (s *StubSource) ReadMessage() (uint8, []byte, error) {
+	return 0, nil, fmt.Errorf("CGO audio not supported on this platform")
+}
+
+// WriteMessage returns an error
+func (s *StubSource) WriteMessage(msgType uint8, payload []byte) error {
+	return fmt.Errorf("CGO audio not supported on this platform")
+}