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>

crates/wzp-native/src/lib.rs

@@ -264,6 +264,12 @@ pub extern "C" fn wzp_native_audio_stop() {
     }
 }
 
+/// Number of capture samples available to read without blocking.
+#[unsafe(no_mangle)]
+pub extern "C" fn wzp_native_audio_capture_available() -> usize {
+    backend().capture.available_read()
+}
+
 /// Read captured PCM samples from the capture ring. Returns the number
 /// of `i16` samples actually copied into `out` (may be less than
 /// `out_len` if the ring is empty).