fix(audio): check capture ring available before read (fixes Opus6k choppy)

Partial reads from the capture ring consumed samples that were then
discarded when the send loop retried from buf[0]. For 20ms codecs this
was invisible (single Oboe burst fills 960 samples in one read), but
40ms codecs (Opus6k, 1920 samples) needed 2 bursts — the first partial
read consumed 960 real samples and threw them away.

Result: Opus6k produced ~11 frames/s instead of 25 (~44% of expected).

Fix: expose wzp_native_audio_capture_available() and check it before
reading, matching the desktop capture_ring.available() pattern. Partial
reads no longer occur because we only read when enough samples exist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/src/engine.rs

@@ -570,16 +570,21 @@ impl CallEngine {
                 if !send_r.load(Ordering::Relaxed) {
                     break;
                 }
-                // wzp-native doesn't expose `available()`, so we just try
-                // to read a full frame and sleep briefly if the ring is
-                // short. Oboe's capture callback fills at a steady rate
-                // so in steady state this spins once per frame.
-                let read = crate::wzp_native::audio_read_capture(&mut buf[..frame_samples]);
-                if read < frame_samples {
+                // Check ring has enough samples before reading to avoid
+                // partial reads that consume samples and then get
+                // overwritten on the next attempt (caused 40ms codecs
+                // like Opus6k to produce ~11 frames/s instead of 25).
+                if crate::wzp_native::audio_capture_available() < frame_samples {
                     short_reads += 1;
                     tokio::time::sleep(std::time::Duration::from_millis(5)).await;
                     continue;
                 }
+                let read = crate::wzp_native::audio_read_capture(&mut buf[..frame_samples]);
+                if read < frame_samples {
+                    // Shouldn't happen after available() check, but guard anyway.
+                    short_reads += 1;
+                    continue;
+                }
                 if !first_full_read_logged {
                     info!(
                         t_ms = send_t0.elapsed().as_millis(),