feat(android): host→client audio — Opus → AAudio (LowLatency)

M4 Android stage 1 (audio). An audio thread pulls Opus packets from the connector
(next_audio), decodes to interleaved f32 stereo, and feeds AAudio via its realtime
data callback through a jitter ring ported from the Linux client (prime ~3 quanta,
drop-oldest cap, re-prime on drain). All in Rust on native threads — symmetric with
the video decode path.

- crates/punktfunk-android: audio.rs (Opus decode + jitter ring + AAudio callback);
  SessionHandle gains an audio slot; nativeStartAudio/nativeStopAudio JNI; Drop stops it.
  Android-only deps: opus 0.3 (libopus via cmake, static) + ndk "audio" (AAudio) — pure
  C/NDK, no libc++_shared to bundle.
- clients/android: NativeBridge start/stop audio, called in the SurfaceView lifecycle.
- kit/build.gradle.kts: cargo-ndk env for the libopus cmake build (NDK root, Ninja,
  LIBOPUS_STATIC/NO_PKG) + --platform 31 (libaaudio is API 26+).

Verified live (emulator -> gamescope host on the LAN box): AAudio opened 48k/stereo/f32;
a 440 Hz tone played into the host capture sink reached the client decoded -- opus ~200/s,
pcm_frames climbing in lockstep, peak=0.089 (real content, not silence), with video
streaming concurrently. Some underruns under emulator jitter (verify on hardware).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/android/app/src/main/kotlin/io/unom/punktfunk/MainActivity.kt

@@ -137,7 +137,8 @@ private fun StreamScreen(handle: Long, onDisconnect: () -> Unit) {
         window?.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
         onDispose {
             window?.clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
-            // Leaving the stream: stop the decode thread and tear down the session.
+            // Leaving the stream: stop the audio + decode threads and tear down the session.
+            NativeBridge.nativeStopAudio(handle)
             NativeBridge.nativeStopVideo(handle)
             NativeBridge.nativeClose(handle)
         }
@@ -152,11 +153,13 @@ private fun StreamScreen(handle: Long, onDisconnect: () -> Unit) {
                 holder.addCallback(object : SurfaceHolder.Callback {
                     override fun surfaceCreated(holder: SurfaceHolder) {
                         NativeBridge.nativeStartVideo(handle, holder.surface)
+                        NativeBridge.nativeStartAudio(handle)
                     }
 
                     override fun surfaceChanged(holder: SurfaceHolder, format: Int, width: Int, height: Int) {}
 
                     override fun surfaceDestroyed(holder: SurfaceHolder) {
+                        NativeBridge.nativeStopAudio(handle)
                         NativeBridge.nativeStopVideo(handle)
                     }
                 })