address review simplifications: setShuffle(seed?) and a nullable playContext

- `QueueRepository.setShuffle(seed: Long?)` derives `shuffleEnabled` from the
  seed, so disabled callers drop the meaningless sentinel `0L`.
- `playContext` returns `Long?` for symmetry with `flatContext`; `anchorPlay`
  treats an out-of-range anchor as -1 instead of silently clamping to the last
  track, and `combinedContextIndexOf` returns null.

app/src/main/kotlin/com/subtracks/data/repo/QueueRepository.kt

@@ -60,8 +60,7 @@ data class QueueSnapshot(
         (
             when {
                 shuffleSeed == null -> upNextAnchor
-                upNextAnchor < 0L -> -1L
-                else -> playContext(upNextAnchor)
+                else -> playContext(upNextAnchor) ?: -1L
             }
         ).coerceAtMost(contextSize - 1L)
 
@@ -80,8 +79,8 @@ data class QueueSnapshot(
         return shuffleSeed?.let { Shuffle.toFlat(it, contextSize, contextPlay) } ?: contextPlay
     }
 
-    fun playContext(flat: Long): Long {
-        if (flat !in 0 until contextSize) return flat
+    fun playContext(flat: Long): Long? {
+        if (flat !in 0 until contextSize) return null
         return shuffleSeed?.let { Shuffle.toSequence(it, contextSize, flat) } ?: flat
     }
 
@@ -208,7 +207,7 @@ class QueueRepository(
     ) {
         write(entries)
         writeUpNext(upNext)
-        setShuffle(shuffleSeed != null, shuffleSeed ?: 0L)
+        setShuffle(shuffleSeed)
         cursorMutex.withLock {
             dao.setCursor(cursorRow().copy(queuePosition = 0, positionMs = 0, upNextAnchor = upNextAnchor))
         }
@@ -239,16 +238,13 @@ class QueueRepository(
         return QueueModes(row.shuffleEnabled, row.repeatMode)
     }
 
-    suspend fun setShuffle(
-        enabled: Boolean,
-        seed: Long,
-    ) {
-        val size = if (enabled) contextSize() else 0L
+    suspend fun setShuffle(seed: Long?) {
+        val size = if (seed != null) contextSize() else 0L
         cursorMutex.withLock {
             dao.setCursor(
                 cursorRow().copy(
-                    shuffleEnabled = enabled,
-                    shuffleSeed = if (enabled) seed else 0L,
+                    shuffleEnabled = seed != null,
+                    shuffleSeed = seed ?: 0L,
                     shuffleSize = size,
                 ),
             )
@@ -288,7 +284,7 @@ class QueueRepository(
     ): Long? {
         val flat = flatIds(snapshot).indexOf(songId)
         if (flat < 0) return null
-        return snapshot.combined(snapshot.playContext(flat.toLong()))
+        return snapshot.playContext(flat.toLong())?.let { snapshot.combined(it) }
     }
 
     suspend fun range(

app/src/main/kotlin/com/subtracks/playback/PlaybackController.kt

@@ -255,7 +255,7 @@ class PlaybackController(
             val start = randomIndex(snap.contextSize, avoid)
             val seed = Random.nextLong()
             val position = Shuffle.toSequence(seed, snap.contextSize, start)
-            queueRepository.setShuffle(true, seed)
+            queueRepository.setShuffle(seed)
             shuffleEnabled = true
             queueSourceId = entry.sourceId
             queueRepository.setCursor(position)
@@ -732,9 +732,9 @@ class PlaybackController(
                 val enabling = !shuffleEnabled
                 val seed = if (enabling) Random.nextLong() else 0L
                 if (enabling) {
-                    queueRepository.setShuffle(true, seed)
+                    queueRepository.setShuffle(seed)
                 } else {
-                    queueRepository.setShuffle(false, 0L)
+                    queueRepository.setShuffle(null)
                 }
                 shuffleEnabled = enabling
                 val reordered = readSnapshot()
@@ -813,7 +813,7 @@ class PlaybackController(
         val position =
             if (shuffleEnabled) {
                 val seed = Random.nextLong()
-                queueRepository.setShuffle(true, seed)
+                queueRepository.setShuffle(seed)
                 Shuffle.toSequence(seed, snapshot.contextSize, start)
             } else {
                 start
@@ -837,7 +837,7 @@ class PlaybackController(
             if (shuffleEnabled && !snapshot.shuffled && snapshot.contextSize > 0L) {
                 val start = snapshot.anchorContextPlay(queueRepository.cursor()).coerceIn(0, snapshot.contextSize - 1)
                 val seed = Random.nextLong()
-                queueRepository.setShuffle(true, seed)
+                queueRepository.setShuffle(seed)
                 queueRepository.setCursor(Shuffle.toSequence(seed, snapshot.contextSize, start))
                 snapshot = queueRepository.snapshot()
             } else if (!snapshot.shuffled) {
@@ -1105,7 +1105,7 @@ class PlaybackController(
     private suspend fun readSnapshot(): QueueSnapshot {
         val snapshot = queueRepository.snapshot()
         if (!snapshot.shuffled) {
-            if (shuffleEnabled && snapshot.size > 0L) queueRepository.setShuffle(false, 0L)
+            if (shuffleEnabled && snapshot.size > 0L) queueRepository.setShuffle(null)
             shuffleEnabled = false
         }
         return snapshot

app/src/test/kotlin/com/subtracks/data/repo/QueueRepositoryTest.kt

@@ -441,7 +441,7 @@ class QueueRepositoryTest {
         runTest {
             seedLibrary()
             val snapshot = repository.snapshotAfter(listOf(repository.albumEntry(1, "al1")))
-            repository.setShuffle(true, 12345L)
+            repository.setShuffle(12345L)
             val shuffled = repository.snapshot()
             val contextBefore = (0 until shuffled.contextSize).mapNotNull { repository.itemAt(shuffled, it)?.song?.id }
 
@@ -545,7 +545,7 @@ class QueueRepositoryTest {
         runTest {
             seedLibrary()
             repository.replace(listOf(repository.albumEntry(1, "al1")))
-            repository.setShuffle(true, 1234L)
+            repository.setShuffle(1234L)
             assertTrue(repository.snapshot().shuffled)
 
             db.libraryDao().upsertSongs(listOf(song("s6", "al1", track = 4, album = "First Album")))
@@ -561,7 +561,7 @@ class QueueRepositoryTest {
             repository.replace(listOf(repository.albumEntry(1, "al1")))
             val entriesBefore = db.queueDao().entries().size
 
-            repository.setShuffle(true, 5678L)
+            repository.setShuffle(5678L)
             val snapshot = repository.snapshot()
 
             assertTrue(snapshot.shuffled)
@@ -584,7 +584,7 @@ class QueueRepositoryTest {
         runTest {
             seedLibrary()
             repository.snapshotAfter(listOf(repository.albumEntry(1, "al1")))
-            repository.setShuffle(true, 42L)
+            repository.setShuffle(42L)
             val shuffled = repository.snapshot()
 
             val resolved = (0 until shuffled.size).mapNotNull { repository.itemAt(shuffled, it)?.song?.id }
@@ -600,10 +600,10 @@ class QueueRepositoryTest {
         runTest {
             seedLibrary()
             repository.snapshotAfter(listOf(repository.albumEntry(1, "al1")))
-            repository.setShuffle(true, 99L)
+            repository.setShuffle(99L)
             assertTrue(repository.snapshot().shuffled)
 
-            repository.setShuffle(false, 0L)
+            repository.setShuffle(null)
 
             val restored = repository.snapshot()
             assertFalse(restored.shuffled)
@@ -615,7 +615,7 @@ class QueueRepositoryTest {
         runTest {
             seedLibrary()
             repository.snapshotAfter(listOf(repository.albumEntry(1, "al1"), repository.songEntry(1, "s4")))
-            repository.setShuffle(true, 42L)
+            repository.setShuffle(42L)
             val snapshot = repository.snapshot()
 
             val range = repository.range(snapshot, 0, snapshot.size - 1)
@@ -630,7 +630,7 @@ class QueueRepositoryTest {
             seedLibrary()
             val snapshot = repository.snapshotAfter(listOf(repository.albumEntry(1, "al1")))
             repository.removeAt(snapshot, 1)
-            repository.setShuffle(true, 7L)
+            repository.setShuffle(7L)
             val shuffled = repository.snapshot()
 
             val range = repository.range(shuffled, 0, shuffled.size - 1)