fix: pre-check hardware rates via /proc/asound before ALSA Direct

When ALSA Direct opens a PCM device and fails to set the rate, the
USB DAC doesn't release the device fast enough for CPAL to open it
afterwards. This caused "device busy" errors even with a delay.

New approach: read /proc/asound/cardN/stream0 to check supported
rates BEFORE opening the device. If the rate isn't supported, skip
ALSA Direct entirely and fall through to CPAL (which has rate
fallback logic). The device is never touched at the wrong rate.

crates/qbz-audio/src/alsa_backend.rs

@@ -262,6 +262,66 @@ fn find_card_number_by_name(short_name: &str) -> Option<String> {
         .map(|c| c.number.clone())
 }
 
+/// Extract card name from an ALSA device ID.
+/// `front:CARD=C20,DEV=0` -> `"C20"`, `hw:0,0` -> card 0 short name, etc.
+fn extract_card_name_from_device(device_id: &str) -> Option<String> {
+    if device_id.starts_with("front:CARD=") || device_id.starts_with("sysdefault:CARD=") {
+        // front:CARD=C20,DEV=0 -> C20
+        let after_card = device_id.split("CARD=").nth(1)?;
+        Some(after_card.split(',').next()?.to_string())
+    } else if device_id.starts_with("hw:") || device_id.starts_with("plughw:") {
+        // hw:0,0 -> card number 0 -> look up short name
+        let prefix = if device_id.starts_with("hw:") { "hw:" } else { "plughw:" };
+        let card_num = device_id.strip_prefix(prefix)?.split(',').next()?;
+        let cards = read_proc_asound_cards();
+        cards.iter()
+            .find(|c| c.number == card_num)
+            .map(|c| c.short_name.clone())
+    } else {
+        None
+    }
+}
+
+/// Read hardware-supported sample rates from /proc/asound/cardN/stream0.
+/// Returns None if rates cannot be determined (treat as "try anyway").
+fn get_hw_supported_rates(card_name: &str) -> Option<Vec<u32>> {
+    let card_num = find_card_number_by_name(card_name)?;
+    let stream_path = format!("/proc/asound/card{}/stream0", card_num);
+    let content = fs::read_to_string(&stream_path).ok()?;
+
+    let mut in_playback = false;
+    let mut all_rates = Vec::new();
+
+    for line in content.lines() {
+        let trimmed = line.trim();
+        if trimmed == "Playback:" {
+            in_playback = true;
+        } else if trimmed == "Capture:" {
+            in_playback = false;
+        }
+        if in_playback && trimmed.starts_with("Rates:") {
+            let rates_str = trimmed.trim_start_matches("Rates:").trim();
+            if rates_str.contains("continuous") {
+                return None; // Any rate supported — don't restrict
+            }
+            for rate_str in rates_str.split(',') {
+                if let Ok(rate) = rate_str.trim().parse::<u32>() {
+                    if !all_rates.contains(&rate) {
+                        all_rates.push(rate);
+                    }
+                }
+            }
+        }
+    }
+
+    if all_rates.is_empty() {
+        None
+    } else {
+        all_rates.sort();
+        Some(all_rates)
+    }
+}
+
 // ============================================================================
 // ALSA Backend Implementation
 // ============================================================================
@@ -463,6 +523,26 @@ impl AlsaBackend {
             (device_id.to_string(), format!("plug:{}", device_id))
         };
 
+        // Pre-check: read /proc/asound/cardN/stream0 to verify rate support
+        // before opening the PCM device. This avoids the "device busy" issue
+        // where ALSA Direct opens the device, fails to configure the rate, and
+        // leaves the device in a state CPAL can't open afterwards.
+        if let Some(card_name) = extract_card_name_from_device(device_id) {
+            if let Some(hw_rates) = get_hw_supported_rates(&card_name) {
+                if !hw_rates.contains(&config.sample_rate) {
+                    log::info!(
+                        "[ALSA Backend] Hardware rates for '{}': {:?}. {}Hz not supported, skipping ALSA Direct",
+                        card_name, hw_rates, config.sample_rate
+                    );
+                    return None;
+                }
+                log::info!(
+                    "[ALSA Backend] Hardware confirms support for {}Hz (card '{}', rates: {:?})",
+                    config.sample_rate, card_name, hw_rates
+                );
+            }
+        }
+
         // Respect ALSA plugin selection from settings
         let try_hw_first = match config.alsa_plugin {
             Some(AlsaPlugin::Hw) => true,