feat(android/diagnostics): per-skip track-change event

Heartbeats are 45s apart and missed a rapid skip burst (local_index
16→22 in one gap). Add a 'playback' track_change event emitted on each
queue-index / current-track change, snapshotting local vs Sonos
index+position + server_health + upnp_loading + route — so a transient
skip-induced desync is captured at the instant it happens. (uiState is a
conflated StateFlow, so a very rapid burst may coalesce intermediate
indices; we still get the boundaries + the snapshot.)

Refs Scribe M9 (#119), task #1210.

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

@@ -102,6 +102,7 @@ class DiagnosticsReporter @Inject constructor(
             launch { collectServerHealth() }
             launch { collectUpnpDrops() }
             launch { collectPlayerState() }
+            launch { collectTrackChanges() }
             launch { collectRoutes() }
             launch { heartbeatLoop() }
         }
@@ -162,6 +163,30 @@ class DiagnosticsReporter @Inject constructor(
             }
     }
 
+    // Emit a snapshot at each track/queue-index change — captures a
+    // skip-induced local↔Sonos desync at the INSTANT it happens, which the
+    // 45s heartbeat misses. Note: uiState is a conflated StateFlow, so a
+    // very rapid skip burst may coalesce intermediate indices (we still get
+    // the boundaries + the local-vs-Sonos snapshot).
+    private suspend fun collectTrackChanges() {
+        playerController.uiState
+            .map { it.queueIndex to it.currentTrack?.id }
+            .distinctUntilChanged()
+            .collect { (index, _) ->
+                val ui = playerController.uiState.value
+                record("playback", buildJsonObject {
+                    put("event", "track_change")
+                    put("local_index", index)
+                    put("local_pos_ms", ui.positionMs)
+                    put("sonos_track", remoteState.trackNumber)
+                    put("sonos_pos_ms", remoteState.positionMs)
+                    put("upnp_loading", ui.isUpnpLoading)
+                    put("server_health", networkStatus.state.value.name)
+                    put("route", outputPicker.routesState.value.current.name)
+                })
+            }
+    }
+
     private suspend fun collectRoutes() {
         // 'playback' — route changes happen for all outputs. This only ever
         // logs the ACTIVE route (routesState.current), so no "connected" flag.