M3d: speaker labels via tinydiarize two-pass merge

- whisper-server stack: second container (port 8086) running the
  English-trained small.en-tdrz model with -tdrz; image patched
  (speaker-turn.patch) to expose speaker_turn_next per segment in
  verbose_json like the cli example does
- core/whisper Diarization: merges the tdrz pass's TURN TIMES onto the
  quality transcript as alternating 'Sprecher 1/2:' labels, splitting
  segments when a turn falls inside them; no turns detected = no
  labels (never mislabels); 6 unit tests
- RemoteWhisperEngine gains a diarize flag (sends tinydiarize=true,
  parses speaker_turn_next); WhisperEngine.Segment carries the flag
- RecorderService: optional second pass on the diarize server after the
  final pass; failures keep the unlabeled transcript
- Settings: Diarize server URL (persisted; empty disables)
- validated infrastructure locally: patched image builds, tdrz model
  downloads from akashmjn/tinydiarize-whisper.cpp, speaker_turn_next
  present in responses; synthetic espeak audio does not trigger the
  model's turn tokens — real two-person speech needed for the
  end-to-end check

android/app/src/main/java/com/meetrec/android/RecordScreen.kt

@@ -67,6 +67,7 @@ fun RecordScreen() {
     val storageUrl = prefs.getString("storage_url", "") ?: ""
     val language = prefs.getString("language", "auto") ?: "auto"
     val liveModel = prefs.getString("live_model", "tiny") ?: "tiny"
+    val diarizeUrl = prefs.getString("diarize_url", "") ?: ""
 
     var agendaText by remember { mutableStateOf(prefs.getString("agenda", "") ?: "") }
     var status by remember { mutableStateOf("Ready.") }
@@ -182,6 +183,7 @@ fun RecordScreen() {
                 },
                 storageUrl = storageUrl.trim().ifBlank { null },
                 agenda = agendaText.lines().map { it.trim() }.filter { it.isNotBlank() },
+                diarizeUrl = diarizeUrl.trim().ifBlank { null },
             )
             if (transcribeOn == "server") {
                 status = "Recording — live and final pass on ${serverUrl.trim()}"

android/app/src/main/java/com/meetrec/android/RecorderService.kt

@@ -15,6 +15,7 @@ import android.util.Log
 import androidx.core.app.NotificationCompat
 import com.meetrec.core.recording.MeetingRecorder
 import com.meetrec.core.recording.WavReader
+import com.meetrec.core.whisper.Diarization
 import com.meetrec.core.whisper.RemoteWhisperEngine
 import com.meetrec.core.whisper.Transcriber
 import com.meetrec.core.whisper.TranscriptFiles
@@ -63,6 +64,8 @@ data class SessionConfig(
     val liveIntervalSec: Int = 8,
     val storageUrl: String? = null,
     val agenda: List<String> = emptyList(),
+    /** tinydiarize server; the final pass is speaker-labeled via its turns. */
+    val diarizeUrl: String? = null,
 )
 
 /** Recording state published to the UI (collect RecorderService.state). */
@@ -294,7 +297,7 @@ class RecorderService : Service() {
                 val samples = withContext(Dispatchers.IO) {
                     file.inputStream().use { WavReader.read(it) }
                 }
-                val segments = transcriber.transcribe(
+                var segments = transcriber.transcribe(
                     samples,
                     WhisperEngine.DEFAULT_THREADS,
                     beamSize = 5,
@@ -302,6 +305,21 @@ class RecorderService : Service() {
                 )
                 Log.i(TAG, "final: ${segments.size} segments in " +
                     "${SystemClock.elapsedRealtime() - t0} ms")
+                // optional speaker labeling: a second pass on the tinydiarize
+                // server contributes only the TURN TIMES, which are merged
+                // onto the quality transcript as "Sprecher 1/2" labels.
+                if (cfg?.diarizeUrl != null) {
+                    try {
+                        val dt0 = SystemClock.elapsedRealtime()
+                        val turns = RemoteWhisperEngine(cfg.diarizeUrl, diarize = true)
+                            .transcribe(samples, WhisperEngine.DEFAULT_THREADS, 5, cfg.language)
+                        segments = Diarization.merge(segments, turns)
+                        Log.i(TAG, "diarize: ${turns.count { it.speakerTurnNext }} turns " +
+                            "in ${SystemClock.elapsedRealtime() - dt0} ms")
+                    } catch (e: Exception) {
+                        Log.w(TAG, "diarize pass failed: ${e.message}")
+                    }
+                }
                 outputs = TranscriptFiles.writeAll(file, durationMs, segments)
                 _state.value = RecordingState.Finished(file, durationMs, sampleRate, segments, outputs)
             } catch (e: Exception) {

android/app/src/main/java/com/meetrec/android/SettingsScreen.kt

@@ -72,6 +72,7 @@ fun SettingsScreen() {
     var langMenu by remember { mutableStateOf(false) }
     var liveModel by remember { mutableStateOf(prefs.getString("live_model", "tiny") ?: "tiny") }
     var liveMenu by remember { mutableStateOf(false) }
+    var diarizeUrl by remember { mutableStateOf(prefs.getString("diarize_url", "") ?: "") }
     var status by remember { mutableStateOf("") }
     var busy by remember { mutableStateOf(false) }
 
@@ -84,6 +85,7 @@ fun SettingsScreen() {
     LaunchedEffect(model) { prefs.edit().putString("model", model).apply() }
     LaunchedEffect(language) { prefs.edit().putString("language", language).apply() }
     LaunchedEffect(liveModel) { prefs.edit().putString("live_model", liveModel).apply() }
+    LaunchedEffect(diarizeUrl) { prefs.edit().putString("diarize_url", diarizeUrl).apply() }
 
     fun <T> runBusy(block: suspend () -> T) {
         if (busy) return
@@ -265,10 +267,20 @@ fun SettingsScreen() {
         Text(status, style = MaterialTheme.typography.bodySmall)
         if (busy) CircularProgressIndicator()
 
+        OutlinedTextField(
+            value = diarizeUrl,
+            onValueChange = { diarizeUrl = it },
+            label = { Text("Diarize server URL") },
+            placeholder = { Text("http://100.103.83.12:8086") },
+            modifier = Modifier.fillMaxWidth(),
+        )
+
         Text(
-            "Transcribe on: phone uses the local engine (download + load), " +
-                "server sends both passes to the whisper.cpp server. Recordings " +
-                "upload to the library when the Library URL is set.",
+            "Speaker labels: when a diarize server is set, a second " +
+                "(English-trained, 2-speaker) tinydiarize pass marks the speaker " +
+                "changes; meetrec merges them onto the transcript as " +
+                "Sprecher 1/2. Best-effort on non-English audio; an empty URL " +
+                "disables it.",
             style = MaterialTheme.typography.bodySmall,
             color = MaterialTheme.colorScheme.onSurfaceVariant,
         )