fix: DirectByteBuffer audio path — eliminate JNI array copies

Adds nativeWriteAudioDirect / nativeReadAudioDirect JNI functions
that accept a DirectByteBuffer instead of ShortArray. The buffer's
native memory is accessed directly by Rust via pointer — no
GetShortArrayRegion / SetShortArrayRegion, no GC-managed array
copies on the audio hot path.

This fixes SIGBUS crashes on Android 16 where ART's concurrent
mark-compact GC crashes when flipping thread roots during JNI
array operations on MAX_PRIORITY audio threads.

Old ShortArray methods kept for backward compatibility.
AudioPipeline switched to use Direct variants.

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

android/app/src/main/java/com/wzp/audio/AudioPipeline.kt

@@ -205,6 +205,8 @@ class AudioPipeline(private val context: Context) {
         Log.i(TAG, "capture started: ${SAMPLE_RATE}Hz mono, buf=$bufSize, aec=${aec?.enabled}, ns=${ns?.enabled}")
 
         val pcm = ShortArray(FRAME_SAMPLES)
+        // DirectByteBuffer for zero-copy JNI (avoids ART GC SIGBUS on Android 16)
+        val directBuf = ByteBuffer.allocateDirect(FRAME_SAMPLES * 2).order(ByteOrder.LITTLE_ENDIAN)
         // Debug: PCM file + RMS CSV
         var pcmOut: BufferedOutputStream? = null
         var rmsCsv: OutputStreamWriter? = null
@@ -224,7 +226,10 @@ class AudioPipeline(private val context: Context) {
                 val read = recorder.read(pcm, 0, FRAME_SAMPLES)
                 if (read > 0) {
                     applyGain(pcm, read, captureGainDb)
-                    engine.writeAudio(pcm)
+                    // Zero-copy write via DirectByteBuffer (no GC array interaction)
+                    directBuf.clear()
+                    directBuf.asShortBuffer().put(pcm, 0, read)
+                    engine.writeAudioDirect(directBuf, read)
 
                     // Debug: write raw PCM + RMS
                     if (pcmOut != null) {
@@ -287,6 +292,8 @@ class AudioPipeline(private val context: Context) {
 
         val pcm = ShortArray(FRAME_SAMPLES)
         val silence = ShortArray(FRAME_SAMPLES)
+        // DirectByteBuffer for zero-copy JNI (avoids ART GC SIGBUS on Android 16)
+        val directBuf = ByteBuffer.allocateDirect(FRAME_SAMPLES * 2).order(ByteOrder.LITTLE_ENDIAN)
         // Debug: PCM file + RMS CSV for playout
         var pcmOut: BufferedOutputStream? = null
         var rmsCsv: OutputStreamWriter? = null
@@ -303,7 +310,13 @@ class AudioPipeline(private val context: Context) {
         }
         try {
             while (running) {
-                val read = engine.readAudio(pcm)
+                // Zero-copy read via DirectByteBuffer
+                directBuf.clear()
+                val read = engine.readAudioDirect(directBuf, FRAME_SAMPLES)
+                if (read > 0) {
+                    directBuf.rewind()
+                    directBuf.asShortBuffer().get(pcm, 0, read)
+                }
                 if (read >= FRAME_SAMPLES) {
                     applyGain(pcm, read, playoutGainDb)
                     track.write(pcm, 0, read)

android/app/src/main/java/com/wzp/engine/WzpEngine.kt

@@ -117,6 +117,26 @@ class WzpEngine(private val callback: WzpCallback) {
         return nativeReadAudio(nativeHandle, pcm)
     }
 
+    /**
+     * Write captured PCM from a DirectByteBuffer — zero JNI array copy.
+     * The buffer must be a direct ByteBuffer with native byte order containing i16 samples.
+     * Called from the AudioRecord capture thread.
+     */
+    fun writeAudioDirect(buffer: java.nio.ByteBuffer, sampleCount: Int): Int {
+        if (nativeHandle == 0L) return 0
+        return nativeWriteAudioDirect(nativeHandle, buffer, sampleCount)
+    }
+
+    /**
+     * Read decoded PCM into a DirectByteBuffer — zero JNI array copy.
+     * The buffer must be a direct ByteBuffer with native byte order.
+     * Called from the AudioTrack playout thread.
+     */
+    fun readAudioDirect(buffer: java.nio.ByteBuffer, maxSamples: Int): Int {
+        if (nativeHandle == 0L) return 0
+        return nativeReadAudioDirect(nativeHandle, buffer, maxSamples)
+    }
+
     // -- JNI native methods --------------------------------------------------
 
     private external fun nativeInit(): Long
@@ -130,6 +150,8 @@ class WzpEngine(private val callback: WzpCallback) {
     private external fun nativeForceProfile(handle: Long, profile: Int)
     private external fun nativeWriteAudio(handle: Long, pcm: ShortArray): Int
     private external fun nativeReadAudio(handle: Long, pcm: ShortArray): Int
+    private external fun nativeWriteAudioDirect(handle: Long, buffer: java.nio.ByteBuffer, sampleCount: Int): Int
+    private external fun nativeReadAudioDirect(handle: Long, buffer: java.nio.ByteBuffer, maxSamples: Int): Int
     private external fun nativeDestroy(handle: Long)
 
     companion object {