perf(audio): skip CPAL probe of virtual ALSA PCMs during enumeration

build_cpal_device_map iterated every PCM that cpal's ALSA host exposes
(dmix:, route:, surround51:, pulse, null, …) but only five name shapes
are ever looked up downstream: default, sysdefault:CARD=*, and
{front,hdmi,iec958}:CARD=*,DEV=*. Dropping the rest avoids holding
Device handles we never query and, combined with the new error
handler, removes another source of spurious libasound probes on
systems where PipeWire owns the hardware exclusively.

cc

crates/qbz-audio/src/alsa_backend.rs

@@ -62,6 +62,19 @@ fn find_best_fallback_rate(requested: u32, supported: &[u32]) -> u32 {
     supported.iter().copied().max().unwrap_or(48000)
 }
 
+/// Return `true` when the given CPAL/ALSA PCM name matches one of the ID
+/// shapes our `/proc/asound`-driven enumeration ever looks up.
+///
+/// Used by `build_cpal_device_map` to drop virtual PCMs (dmix, route,
+/// surround*, pulse, null, …) whose probing only produces noise.
+fn is_known_pcm_id(name: &str) -> bool {
+    name == "default"
+        || name.starts_with("sysdefault:CARD=")
+        || name.starts_with("front:CARD=")
+        || name.starts_with("hdmi:CARD=")
+        || name.starts_with("iec958:CARD=")
+}
+
 /// Extract supported sample rates from a CPAL device
 fn get_supported_sample_rates(device: &rodio::cpal::Device) -> Option<Vec<u32>> {
     use rodio::cpal::traits::DeviceTrait;
@@ -546,15 +559,26 @@ impl AlsaBackend {
         Ok(devices)
     }
 
-    /// Build a map of device_id -> CPAL Device for sample rate queries
-    /// This is OPTIONAL enrichment - devices may be missing if in exclusive use
+    /// Build a map of device_id -> CPAL Device for sample rate queries.
+    /// This is OPTIONAL enrichment — devices may be missing if in exclusive use.
+    ///
+    /// Only the PCM name patterns we actually look up downstream are kept:
+    /// `default`, `sysdefault:CARD=…`, and `{front,hdmi,iec958}:CARD=…,DEV=…`.
+    /// Virtual PCMs (`dmix:`, `route:`, `surround51:` and the like) are
+    /// dropped — we never query them, and letting them reach a later
+    /// `supported_output_configs()` call just invites spurious libasound
+    /// errors ("unable to open slave", "no matching channel map") on systems
+    /// where PipeWire or another client holds the underlying hardware.
     fn build_cpal_device_map(&self) -> HashMap<String, rodio::cpal::Device> {
         let mut map = HashMap::new();
 
         if let Ok(output_devices) = self.host.output_devices() {
             for device in output_devices {
                 if let Ok(description) = device.description() {
-                    map.insert(description.name().to_string(), device);
+                    let name = description.name().to_string();
+                    if is_known_pcm_id(&name) {
+                        map.insert(name, device);
+                    }
                 }
             }
         }