perf(android): gate audio prefetch on isPlaying

Cold-start playback on a fresh install was taking ~25 s before any
audio played. Logcat showed AudioPrefetcher was kicking off N
concurrent CacheWriter jobs the instant setQueue updated uiState --
each prefetch a full upcoming-track download over the same OkHttp
client as the current-track DataSource. Five-way bandwidth split
plus parallel Coil cover fetches starved the current track until
its full file body had streamed through (~12 MB at ~1 MB/s under
contention).

Now reconcile() observes uiState.isPlaying and starts upcoming-track
prefetches only when the current track is actually playing.
Cancellation of out-of-window jobs always runs so a queue switch or
skip still frees the pipe immediately, even while paused. Cold start
should drop from ~25 s -> 5-7 s on the user's network: just the
single-stream throughput plus the one-time TLS/DNS tax.

Refactored the inline reconcile body into computeTargets /
cancelOutOfWindowLocked / startInWindowLocked helpers to keep
ReturnCount under the detekt cap.

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

@@ -58,59 +58,98 @@ class AudioPrefetcher @Inject constructor(
     private val activeJobs = mutableMapOf<String, Job>()
     private val mutex = Mutex()
 
+    private data class ReconcileInput(
+        val queue: List<Pair<String, String>>,
+        val index: Int,
+        val window: Int,
+        val isPlaying: Boolean,
+    )
+
     init {
         scope.launch {
             combine(
                 playerController.uiState.map { it.queue.map { t -> t.id to t.streamUrl } },
                 playerController.uiState.map { it.queueIndex },
                 authStore.cacheSettings.map { it.prefetchWindow },
-            ) { queue, index, window -> Triple(queue, index, window) }
+                playerController.uiState.map { it.isPlaying },
+            ) { queue, index, window, isPlaying ->
+                ReconcileInput(queue, index, window, isPlaying)
+            }
                 .distinctUntilChanged()
-                .collect { (queue, index, window) -> reconcile(queue, index, window) }
+                .collect { input ->
+                    reconcile(input.queue, input.index, input.window, input.isPlaying)
+                }
         }
     }
 
+    /**
+     * Apply the prefetch window to the current queue.
+     *
+     * Cancellation of out-of-window jobs always runs -- a queue mutation
+     * or skip should free bandwidth from stale prefetches immediately.
+     * Starting new prefetches is gated on [isPlaying]: until the current
+     * track is actually playing, every byte of upstream bandwidth should
+     * land on it, not on upcoming-track prefetches. Without this gate a
+     * cold start fanned out 4-6 concurrent CacheWriter jobs against the
+     * same OkHttp client as the playback DataSource and the user waited
+     * ~25 s for the first audio to start; with the gate the current
+     * track gets the full pipe to its first STATE_READY, then the
+     * prefetcher fills in the next window.
+     */
     private suspend fun reconcile(
         queue: List<Pair<String, String>>,
         index: Int,
         window: Int,
+        isPlaying: Boolean,
     ) {
         mutex.withLock {
-            if (index < 0 || queue.isEmpty() || window <= 0) {
+            val targets = computeTargets(queue, index, window)
+            if (targets.isEmpty()) {
                 cancelAllLocked()
                 return
             }
-            // Exclude the currently-playing track (it's loaded by the
-            // player itself) and walk `window` tracks forward.
-            val firstIdx = (index + 1).coerceAtMost(queue.size)
-            val lastIdx = (index + window).coerceAtMost(queue.size - 1)
-            if (firstIdx > lastIdx) {
-                cancelAllLocked()
-                return
-            }
-            val targets = queue.subList(firstIdx, lastIdx + 1)
             val targetIds = targets.mapTo(mutableSetOf()) { it.first }
+            // Cancellation always runs so a queue mutation or skip frees
+            // the pipe immediately, even while paused.
+            cancelOutOfWindowLocked(targetIds)
+            if (isPlaying) startInWindowLocked(targets)
+        }
+    }
 
-            // Cancel jobs for tracks that have slid out of the window.
-            activeJobs.entries
-                .filter { it.key !in targetIds }
-                .toList()
-                .forEach { (id, job) ->
-                    job.cancel()
-                    activeJobs.remove(id)
-                }
+    private fun computeTargets(
+        queue: List<Pair<String, String>>,
+        index: Int,
+        window: Int,
+    ): List<Pair<String, String>> {
+        // Exclude the currently-playing track (it's loaded by the player
+        // itself) and walk `window` tracks forward.
+        val firstIdx = index + 1
+        val lastIdx = (index + window).coerceAtMost(queue.size - 1)
+        val isValid = index >= 0 && queue.isNotEmpty() && window > 0 && firstIdx <= lastIdx
+        return if (isValid) queue.subList(firstIdx, lastIdx + 1) else emptyList()
+    }
 
-            // Start prefetches for new arrivals. Skip blank URLs (these
-            // come from minimal TrackRefs synthesized from playlist rows
-            // when the upstream track was removed from the library).
-            for ((trackId, streamUrl) in targets) {
-                if (trackId in activeJobs || streamUrl.isBlank()) continue
-                val job = scope.launch(Dispatchers.IO) {
-                    runCatching { prefetchOne(trackId, streamUrl) }
-                    mutex.withLock { activeJobs.remove(trackId) }
-                }
-                activeJobs[trackId] = job
+    private fun cancelOutOfWindowLocked(targetIds: Set<String>) {
+        activeJobs.entries
+            .filter { it.key !in targetIds }
+            .toList()
+            .forEach { (id, job) ->
+                job.cancel()
+                activeJobs.remove(id)
             }
+    }
+
+    private fun startInWindowLocked(targets: List<Pair<String, String>>) {
+        // Skip blank URLs (these come from minimal TrackRefs synthesized
+        // from playlist rows when the upstream track was removed from
+        // the library).
+        for ((trackId, streamUrl) in targets) {
+            if (trackId in activeJobs || streamUrl.isBlank()) continue
+            val job = scope.launch(Dispatchers.IO) {
+                runCatching { prefetchOne(trackId, streamUrl) }
+                mutex.withLock { activeJobs.remove(trackId) }
+            }
+            activeJobs[trackId] = job
         }
     }