fix: suspend PipeWire before CPAL stream in exclusive mode

When transitioning from ALSA Direct to CPAL fallback (e.g. for rate
resampling), PipeWire auto-re-acquires the device after ALSA Direct
releases it. CPAL then can't open the device.

Suspend PipeWire sinks before creating CPAL stream in exclusive mode,
same pattern used in ALSA Direct DeviceBusy handler.

crates/qbz-audio/src/alsa_backend.rs

@@ -876,6 +876,24 @@ impl AudioBackend for AlsaBackend {
             SampleFormat::F32,
         );
 
+        // In exclusive mode, PipeWire may have re-acquired the device after the
+        // previous ALSA Direct stream released it. Suspend PipeWire before opening.
+        if config.exclusive_mode {
+            log::info!("[ALSA Backend] Exclusive mode: suspending PipeWire sinks before CPAL stream");
+            if let Ok(output) = std::process::Command::new("pactl")
+                .args(["suspend-sink", "@DEFAULT_SINK@", "1"])
+                .output()
+            {
+                if output.status.success() {
+                    log::info!("[ALSA Backend] PipeWire sink suspended");
+                    std::thread::sleep(std::time::Duration::from_millis(200));
+                } else {
+                    let stderr = String::from_utf8_lossy(&output.stderr);
+                    log::warn!("[ALSA Backend] Failed to suspend PipeWire sink: {}", stderr);
+                }
+            }
+        }
+
         // Create MixerDeviceSink with custom config
         let mixer_sink = DeviceSinkBuilder::from_device(device)
             .map_err(|e| {