feat(android): SetNextAVTransportURI pre-queue for gap-free UPnP advance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/player/MinstrelForwardingPlayer.kt

@@ -52,6 +52,15 @@ class MinstrelForwardingPlayer(
         scope.launch {
             holder.active.collect { active -> onActiveChanged(active) }
         }
+        delegate.addListener(object : Player.Listener {
+            override fun onMediaItemTransition(
+                mediaItem: androidx.media3.common.MediaItem?,
+                reason: Int,
+            ) {
+                val active = holder.active.value ?: return
+                scope.launch { preQueueNext(active) }
+            }
+        })
     }
 
     private fun isRemote(): Boolean = holder.active.value != null
@@ -159,6 +168,34 @@ class MinstrelForwardingPlayer(
         }.onFailure { handleSoapFailure(active, it) }
     }
 
+    /**
+     * Speculative pre-queue of the next track so Sonos can advance gap-free.
+     * Failures (token mint OR SetNextAVTransportURI rejection) silently fall
+     * back to the poll-driven advance path -- the pollLoop detects STOPPED +
+     * 0:00 RelTime and calls syncCurrentItemToRemote on whatever the new
+     * currentMediaItem is. The only consequence of failure here is a sub-second
+     * audible gap at track boundary, not a broken route.
+     */
+    private suspend fun preQueueNext(active: ActiveUpnp) {
+        val nextMediaId = handlerEvaluate { nextItemMediaId() } ?: return
+        val token = runCatching { tokens.mint(nextMediaId) }.getOrNull() ?: return
+        runCatching {
+            active.avTransport.setNextAVTransportURI(
+                uri = token.url,
+                mime = token.mime,
+                title = token.title,
+            )
+        }.onFailure {
+            Timber.w(it, "SetNextAVTransportURI failed (poll-driven advance will fall back)")
+        }
+    }
+
+    private fun nextItemMediaId(): String? {
+        val nextIdx = delegate.nextMediaItemIndex
+        if (nextIdx == androidx.media3.common.C.INDEX_UNSET) return null
+        return delegate.getMediaItemAt(nextIdx).mediaId
+    }
+
     private fun handleSoapFailure(active: ActiveUpnp, t: Throwable) {
         pollJob?.cancel()
         Timber.w(t, "UPnP transport call failed on %s", active.routeName)