fix: monotonic playGeneration instead of re-settable stopped flag

stopped flag was set before join() but cleared after, causing race:
if thread was still blocked in decodeRead when stopped was reset,
onComplete fired anyway. Monotonic playGeneration ++ never decreases,
thread captures myGen at start, compares after completion —
correctly skipped if generation changed (another stop/seek started).

tools/chatgui-android/app/src/main/java/com/utun/chat/data/AudioRecorderManager.kt

@@ -185,7 +185,7 @@ class AudioRecorderManager {
     @Volatile var elapsedMs: Int = 0; private set
     var pausedElapsedMs: Int = 0; private set
     private var playThread: Thread? = null
-    @Volatile private var stopped = false
+    @Volatile private var playGeneration = 0
 
     fun playVoiceFile(filePath: String, onProgress: ((Float) -> Unit)? = null, onComplete: () -> Unit = {}) {
         playFromPosition(filePath, 0, onProgress, onComplete)
@@ -196,6 +196,7 @@ class AudioRecorderManager {
         isPaused = false; elapsedMs = startMs; playing = true
 
         playThread = Thread {
+            val myGen = playGeneration
             try {
                 val info = IntArray(3)
                 val handle = NativeLib.voiceDecodeOpen(filePath, info)
@@ -246,7 +247,7 @@ class AudioRecorderManager {
             } catch (e: Exception) {
                 LogManager.addLog("ERROR", "AudioPlayer", "play error: ${e.message}")
             }
-            if (!stopped) {
+            if (playGeneration == myGen) {
                 playing = false; isPaused = false; onComplete()
             }
         }.apply { start() }
@@ -254,14 +255,12 @@ class AudioRecorderManager {
 
     fun pausePlayback() {
         pausedElapsedMs = elapsedMs
-        stopped = true; isPaused = true; playing = false
+        playGeneration++; isPaused = true; playing = false
         playThread?.join(500); playThread = null
-        stopped = false
     }
 
     fun stopPlayback() {
-        stopped = true; playing = false; isPaused = false; pausedElapsedMs = 0
+        playGeneration++; playing = false; isPaused = false; pausedElapsedMs = 0
         playThread?.join(500); playThread = null
-        stopped = false
     }
 }