replace only the queue items a delete actually changed

Deleting any download re-set the whole queue window, which makes the player
re-prepare the current item and audibly pauses. It now rebuilds only the items
whose local file appeared or went away, comparing what the window reports with
what the repository can resolve, and falls back to the full reload when the
window no longer lines up.

app/src/test/kotlin/com/subtracks/playback/FakePlayer.kt

@@ -61,6 +61,20 @@ class FakePlayerHandle : PlayerHandle {
 
     override val currentItem: QueueItem? get() = items.getOrNull(index)
 
+    /** Overrides the URIs the window reports; defaults to a stream URI per item. */
+    @Volatile
+    var uris: List<String> = emptyList()
+
+    override fun itemUris(): List<String?> = uris.ifEmpty { items.map { "http://stream/${it.id}" } }
+
+    override fun replaceItem(
+        index: Int,
+        item: QueueItem,
+    ) {
+        operations += "replaceItem($index, ${item.id})"
+        if (index in items.indices) items[index] = item
+    }
+
     override fun setWindow(
         items: List<QueueItem>,
         startIndex: Int,

app/src/test/kotlin/com/subtracks/playback/PlaybackControllerTest.kt

@@ -2300,7 +2300,7 @@ class PlaybackControllerTest {
     }
 
     @Test
-    fun refreshingMediaItemsRebuildsTheWindowAtTheSamePosition() {
+    fun refreshingMediaItemsLeavesAnUnaffectedQueueAlone() {
         seedAlbum(3, sourceId = 1)
         controller.playAlbum(1, "al1", 0)
         await { handle.operations.contains("play") }
@@ -2308,12 +2308,28 @@ class PlaybackControllerTest {
         val windows = handle.operations.count { it.startsWith("setWindow") }
 
         controller.refreshMediaItems()
+        Thread.sleep(200)
 
-        await { handle.operations.count { it.startsWith("setWindow") } > windows }
+        assertEquals(windows, handle.operations.count { it.startsWith("setWindow") })
         assertEquals("s1", handle.currentItem?.id)
         assertEquals(42_000L, handle.positionMs)
     }
 
+    @Test
+    fun refreshingMediaItemsReplacesOnlyAnItemWhoseLocalFileIsGone() {
+        seedAlbum(3, sourceId = 1)
+        controller.playAlbum(1, "al1", 0)
+        await { handle.operations.contains("play") }
+        val windows = handle.operations.count { it.startsWith("setWindow") }
+        handle.uris = handle.items.mapIndexed { index, item -> if (index == 0) "file:///gone" else "http://stream/${item.id}" }
+
+        controller.refreshMediaItems()
+
+        await { handle.operations.any { it.startsWith("replaceItem(0,") } }
+        assertEquals(windows, handle.operations.count { it.startsWith("setWindow") })
+        assertEquals(0, handle.operations.count { it.startsWith("replaceItem(1,") })
+    }
+
     @Test
     fun positionTickerAdvancesPositionWithoutEmittingPlaybackState() =
         runBlocking {