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)
                     }
                 })

clients/android/kit/build.gradle.kts

@@ -56,14 +56,29 @@ fun registerCargoNdk(taskName: String, release: Boolean) =
         description = "cargo-ndk build of punktfunk-android (${if (release) "release" else "debug"})"
         workingDir = repoRoot
         val sdk = androidSdkDir()
-        // A GUI Android Studio launch does not source the login shell, so make cargo + the NDK
-        // discoverable explicitly (works the same from a bare CLI).
-        environment("PATH", cargoBin + File.pathSeparator + System.getenv("PATH"))
+        // A GUI Android Studio launch does not source the login shell, so make cargo, the NDK, and
+        // cmake (libopus builds via the cmake crate) discoverable explicitly — same as a bare CLI.
+        val cmakeBin = "$sdk/cmake/3.22.1/bin"
+        environment(
+            "PATH",
+            cargoBin + File.pathSeparator + cmakeBin + File.pathSeparator + System.getenv("PATH"),
+        )
         environment("ANDROID_HOME", sdk)
         environment("ANDROID_NDK_HOME", "$sdk/ndk/$ndkVer")
+        // CMake's built-in Android support (used by the cmake crate for libopus) finds the NDK via
+        // these, and uses Ninja (bundled next to the SDK cmake) since there's no `make`.
+        environment("ANDROID_NDK_ROOT", "$sdk/ndk/$ndkVer")
+        environment("ANDROID_NDK", "$sdk/ndk/$ndkVer")
+        environment("CMAKE_GENERATOR", "Ninja")
+        // audiopus_sys picks static-vs-dynamic by HOST not target — force the bundled static libopus
+        // (pure C) so the android .so links it instead of looking for the host's libopus.so.
+        environment("LIBOPUS_STATIC", "1")
+        environment("LIBOPUS_NO_PKG", "1")
         val cmd = mutableListOf(
             "cargo", "ndk",
             "-t", "arm64-v8a", "-t", "x86_64",
+            // Link against the minSdk-31 sysroot so libaaudio (API 26+) is found.
+            "--platform", "31",
             "-o", file("src/main/jniLibs").absolutePath,
             "build", "-p", "punktfunk-android",
         )

clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt

@@ -37,4 +37,13 @@ object NativeBridge {
 
     /** Stop + join the decode thread without closing the session. No-op on `0`. */
     external fun nativeStopVideo(handle: Long)
+
+    /**
+     * Start host→client audio: Opus decode → jitter ring → AAudio (LowLatency), all in Rust. No-op
+     * if already started. Best-effort — a failure leaves video streaming.
+     */
+    external fun nativeStartAudio(handle: Long)
+
+    /** Stop + join the audio thread and close AAudio, without closing the session. No-op on `0`. */
+    external fun nativeStopAudio(handle: Long)
 }