stop locating the playing queue entry by song id

The queue can hold the same song more than once (a song in both the context
and the up-next block, a duplicate album track, ...), so resolving the
playing entry by song id could match the wrong copy.

- `PlaybackController.move` and `removeAt` now derive the current track's
  new position from the edit (block index / context play, read back through
  the updated snapshot so a moved anchor is accounted for) instead of
  `combinedIndexOf`/`combinedContextIndexOf`.
- the queue view's playing highlight compares the row position to the
  playback position instead of the song id, so duplicates no longer both
  light up.

Regression tests cover a move and a remove with a duplicate song in the
context; both fail without the change.

Still id-based, deliberately: `undo`'s canMirror guard (already position-
guarded), the shuffle-state-changed branch of `shiftWindowLocked` (rare,
needs the id to find the track after an arbitrary reorder) and the view's
LazyColumn keys (cosmetic only).

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

@@ -490,6 +490,42 @@ class PlaybackControllerTest {
         }
     }
 
+    @Test
+    fun movingWithADuplicateInTheContextKeepsThePlayingCopy() {
+        seedAlbum(4, sourceId = 1)
+        runBlocking { queues.replace(listOf(queues.albumEntry(1, "al1"), queues.songEntry(1, "s2"))) }
+        controller.connect()
+        await { controller.state.value.item != null }
+        // Context is s1,s2,s3,s4,s2; play the second copy.
+        controller.playAt(4)
+        await { controller.state.value.position == 4L }
+
+        runBlocking { controller.move(0, 3) }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        val playing = controller.state.value.position!!
+        assertTrue(runBlocking { queues.itemAt(snapshot, playing)?.song?.id } == "s2")
+        // Still the second copy (flat index 4), not the first.
+        assertEquals(4L, snapshot.flatContext(snapshot.contextPlay(playing)))
+    }
+
+    @Test
+    fun removingAnotherTrackWithADuplicateInTheContextKeepsThePlayingCopy() {
+        seedAlbum(4, sourceId = 1)
+        runBlocking { queues.replace(listOf(queues.albumEntry(1, "al1"), queues.songEntry(1, "s2"))) }
+        controller.connect()
+        await { controller.state.value.item != null }
+        controller.playAt(4)
+        await { controller.state.value.position == 4L }
+
+        runBlocking { controller.removeAt(0) }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        val playing = controller.state.value.position!!
+        assertTrue(runBlocking { queues.itemAt(snapshot, playing)?.song?.id } == "s2")
+        assertEquals(3L, snapshot.flatContext(snapshot.contextPlay(playing)))
+    }
+
     @Test
     fun playNextFromInsideTheBlockGoesAfterTheCurrentTrack() {
         seedAlbum(3, sourceId = 1)
@@ -1637,6 +1673,29 @@ class PlaybackControllerTest {
         )
     }
 
+    @Test
+    fun removingThePlayingAnchorWhileShuffledPlaysTheQueuedTrackNext() {
+        seedAlbum(6, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 2)
+        await {
+            controller.state.value.item
+                ?.id == "s3"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        controller.toggleShuffle()
+        await { controller.state.value.shuffle }
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+        val current = controller.state.value.position!!
+
+        runBlocking { controller.removeAt(current) }
+
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+    }
+
     @Test
     fun removingThePlayingTrackWhileShuffledLeavesTheBlockReachable() {
         seedAlbum(6, sourceId = 1)

app/src/test/kotlin/com/subtracks/ui/AppScreenshotTest.kt

@@ -415,7 +415,7 @@ class AppScreenshotTest {
                     rows =
                         Fixtures.songItems.mapIndexed { index, item -> QueueRow(index.toLong(), index.toLong(), item) },
                     ready = true,
-                    currentSongId = Fixtures.songItems[1].song.id,
+                    currentPosition = 1L,
                     coverArt = { id, _ -> id?.let { CoverArtRef(it, "test:$it") } },
                     onBack = {},
                     onPlay = {},