fix: AudioRing cursor desync + capture thread use-after-free

AudioRing (reader-detects-lap architecture):
- Writer NEVER touches read_pos — fixes SPSC invariant violation
- Reader self-corrects when lapped (snaps read_pos forward)
- Power-of-2 capacity (16384 = 341ms) with bitmask indexing
- Added overflow_count and underrun_count diagnostics
- Wired ring health into engine stats and periodic logging

Capture thread use-after-free (drain latch):
- Added CountDownLatch(2) to AudioPipeline
- Audio threads count down after exiting their loops
- teardown() awaits latch (200ms timeout) before destroy()
- Guarantees no in-flight JNI calls when native handle is freed
- stopAudio() no longer nulls pipeline (teardown handles it)

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

android/app/src/main/java/com/wzp/ui/call/CallViewModel.kt

@@ -254,8 +254,17 @@ class CallViewModel : ViewModel(), WzpCallback {
         Log.i(TAG, "teardown: stopping audio, stopService=$stopService")
         val hadCall = audioStarted
         CallService.onStopFromNotification = null
-        stopAudio()
+        stopAudio()             // sets running=false (non-blocking)
         stopStatsPolling()
+
+        // Wait for audio threads to exit their loops before destroying the engine.
+        // This guarantees no in-flight JNI calls to writeAudio/readAudio.
+        val drained = audioPipeline?.awaitDrain() ?: true
+        if (!drained) {
+            Log.w(TAG, "teardown: audio threads did not drain in time")
+        }
+        audioPipeline = null
+
         Log.i(TAG, "teardown: stopping engine")
         try { engine?.stopCall() } catch (e: Exception) { Log.w(TAG, "stopCall err: $e") }
         try { engine?.destroy() } catch (e: Exception) { Log.w(TAG, "destroy err: $e") }
@@ -399,8 +408,7 @@ class CallViewModel : ViewModel(), WzpCallback {
 
     private fun stopAudio() {
         if (!audioStarted) return
-        audioPipeline?.stop()
-        audioPipeline = null
+        audioPipeline?.stop()    // sets running=false; DON'T null — teardown needs awaitDrain()
         audioRouteManager?.unregister()
         audioRouteManager?.setSpeaker(false)
         _isSpeaker.value = false