fix(qbz-audio): use alsa::pcm::Frames for ALSA buffer and period sizes

HwParams::set_buffer_size_near and set_period_size_near take alsa::pcm::Frames, which is libc::c_long: i64 on 64-bit targets but i32 on 32-bit ones. The three buffer_size computations in alsa_direct.rs cast to i64 explicitly, so the crate does not compile for any 32-bit target.

Casting to the alsa type instead is a no-op on 64-bit, where Frames already is i64, and correct on 32-bit. No behavioural change on any currently supported target: every value involved is a sample rate divided by 2, 4 or 8, so at most 96000, far below an i32 overflow.

Verified with cargo check -p qbz-audio:
  x86_64-unknown-linux-gnu       passes before and after
  arm-unknown-linux-gnueabihf    6 x E0308 before, passes after

crates/qbz-audio/src/alsa_direct.rs

@@ -4,7 +4,7 @@
 //! This module bypasses rodio/CPAL completely for direct hardware access.
 
 #[cfg(target_os = "linux")]
-use alsa::pcm::{Access, Format, HwParams, PCM};
+use alsa::pcm::{Access, Format, Frames, HwParams, PCM};
 #[cfg(target_os = "linux")]
 use alsa::{Direction, ValueOr};
 #[cfg(target_os = "linux")]
@@ -213,13 +213,13 @@ impl AlsaDirectStream {
             // Set buffer size (larger buffer for high-res audio)
             let buffer_size = if sample_rate >= 192000 {
                 // 500ms buffer for 192kHz+ (like MPD config)
-                (sample_rate / 2) as i64
+                (sample_rate / 2) as Frames
             } else if sample_rate >= 96000 {
                 // 250ms buffer for 96kHz
-                (sample_rate / 4) as i64
+                (sample_rate / 4) as Frames
             } else {
                 // 125ms buffer for lower rates
-                (sample_rate / 8) as i64
+                (sample_rate / 8) as Frames
             };
 
             hwp.set_buffer_size_near(buffer_size)
@@ -300,11 +300,11 @@ impl AlsaDirectStream {
             hwp.set_rate(carrier_rate, ValueOr::Nearest)
                 .map_err(|e| format!("Failed to set DoP carrier rate {}: {}", carrier_rate, e))?;
             let buffer_size = if carrier_rate >= 192000 {
-                (carrier_rate / 2) as i64
+                (carrier_rate / 2) as Frames
             } else if carrier_rate >= 96000 {
-                (carrier_rate / 4) as i64
+                (carrier_rate / 4) as Frames
             } else {
-                (carrier_rate / 8) as i64
+                (carrier_rate / 8) as Frames
             };
             hwp.set_buffer_size_near(buffer_size)
                 .map_err(|e| format!("Failed to set buffer size: {}", e))?;
@@ -388,7 +388,7 @@ impl AlsaDirectStream {
                 .map_err(|e| format!("Failed to set channels: {}", e))?;
             hwp.set_rate(rate, ValueOr::Nearest)
                 .map_err(|e| format!("Failed to set native DSD rate {}: {}", rate, e))?;
-            let buffer_size = (rate / 4) as i64; // 250 ms
+            let buffer_size = (rate / 4) as Frames; // 250 ms
             hwp.set_buffer_size_near(buffer_size)
                 .map_err(|e| format!("Failed to set buffer size: {}", e))?;
             hwp.set_period_size_near(buffer_size / 10, ValueOr::Nearest)