feat(android/diagnostics): track-identity enrichment + zero stale Sonos state

Numeric indices wobble across re-casts (offset +1↔0 seen during output
toggling), making "same track?" ambiguous. Enrich both the track_change
event and the heartbeat with local_track_id (TrackRef.id) and sonos_uri
(RemotePlayerState.currentTrackUri — the URL the speaker is actually
streaming), so a desync is unambiguous.

Also fixes the cast→phone stale-state pollution (#1211): sonos_* is now
zeroed unless a remote route is active, via a shared putSonos() helper —
so a just-ended cast's RemotePlayerState can't masquerade as live Sonos
data in the diagnostics.

Refs Scribe M9 (#119), tasks #1210 #1211.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K55iTxn95BtshocgdE1shW

android/app/src/main/java/com/fabledsword/minstrel/diagnostics/DiagnosticsReporter.kt

@@ -174,12 +174,16 @@ class DiagnosticsReporter @Inject constructor(
             .distinctUntilChanged()
             .collect { (index, _) ->
                 val ui = playerController.uiState.value
+                val casting = outputPicker.routesState.value.current.protocol !=
+                    OutputRoute.Protocol.SYSTEM
                 record("playback", buildJsonObject {
                     put("event", "track_change")
                     put("local_index", index)
+                    // Track IDENTITY, not just index — indices wobble across
+                    // re-casts, so the id + Sonos URI make a desync unambiguous.
+                    put("local_track_id", ui.currentTrack?.id ?: "")
                     put("local_pos_ms", ui.positionMs)
-                    put("sonos_track", remoteState.trackNumber)
-                    put("sonos_pos_ms", remoteState.positionMs)
+                    putSonos(this, casting)
                     put("upnp_loading", ui.isUpnpLoading)
                     put("server_health", networkStatus.state.value.name)
                     put("route", outputPicker.routesState.value.current.name)
@@ -214,13 +218,11 @@ class DiagnosticsReporter @Inject constructor(
                 put("is_playing", ui.isPlaying)
                 put("source", ui.currentSource ?: "")
                 put("local_index", ui.queueIndex)
+                put("local_track_id", ui.currentTrack?.id ?: "")
                 put("local_pos_ms", ui.positionMs)
                 put("route", route.name)
                 put("route_protocol", route.protocol.name)
-                // UPnP/Sonos remote-vs-local — the desync signal.
-                put("sonos_track", remoteState.trackNumber)
-                put("sonos_pos_ms", remoteState.positionMs)
-                put("sonos_playing", remoteState.isPlaying)
+                putSonos(this, routeActive)
                 addPowerFields(this)
             })
         }
@@ -285,6 +287,18 @@ class DiagnosticsReporter @Inject constructor(
         addPowerFields(this)
     }
 
+    // Sonos/UPnP remote-vs-local desync fields. Only meaningful while a
+    // remote route is active; zeroed otherwise so a stale RemotePlayerState
+    // from a just-ended cast can't masquerade as live Sonos data (the
+    // cast→phone handoff artifact). currentTrackUri is the desync ground
+    // truth — it carries the track the speaker is actually streaming.
+    private fun putSonos(builder: kotlinx.serialization.json.JsonObjectBuilder, casting: Boolean) {
+        builder.put("sonos_track", if (casting) remoteState.trackNumber else 0)
+        builder.put("sonos_pos_ms", if (casting) remoteState.positionMs else 0)
+        builder.put("sonos_playing", casting && remoteState.isPlaying)
+        if (casting) builder.put("sonos_uri", remoteState.currentTrackUri)
+    }
+
     private fun addPowerFields(builder: kotlinx.serialization.json.JsonObjectBuilder) {
         val pm = context.getSystemService(Context.POWER_SERVICE) as PowerManager
         builder.put("doze", pm.isDeviceIdleMode)