playback: share edge wrapping with the art predictions

Lift the wrap arithmetic out of next(), previous() and previousTrack() into nextPosition()/previousPosition() so upcomingItem()/previousItem() cannot drift from the actions again. A one-track queue no longer predicts itself as its own neighbour, online or offline: the wrap would land on the current position, and the actions are a no-op there, so the strip offered a dead swipe.

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

@@ -2262,6 +2262,38 @@ class PlaybackControllerTest {
         )
     }
 
+    @Test
+    fun aSingleTrackQueueHasNoAdjacentArtwork() {
+        seedAlbum(1, sourceId = 1)
+        controller.playAlbum(1, "al1", 0)
+        await {
+            controller.state.value.item
+                ?.id == "s1"
+        }
+        controller.cycleRepeat()
+        assertEquals(RepeatMode.All, controller.state.value.repeat)
+
+        assertNull(runBlocking { controller.upcomingItem() })
+        assertNull(runBlocking { controller.previousItem() })
+    }
+
+    @Test
+    fun aSingleDownloadedTrackHasNoAdjacentArtworkWhenOffline() {
+        seedAlbum(1, sourceId = 1)
+        markDownloaded(1, "s1")
+        setOffline()
+        controller.playAlbum(1, "al1", 0)
+        await {
+            controller.state.value.item
+                ?.id == "s1"
+        }
+        controller.cycleRepeat()
+        assertEquals(RepeatMode.All, controller.state.value.repeat)
+
+        assertNull(runBlocking { controller.upcomingItem() })
+        assertNull(runBlocking { controller.previousItem() })
+    }
+
     @Test
     fun repeatAllLoopsAfterEndingWithRepeatOff() {
         seedAlbum(2, sourceId = 1)