address review: cancellation-safe star, one star handler, focus logic tested

- setStar no longer treats cancellation as a rejection: rethrow CancellationException
  instead of toasting/rolling back, and roll back under NonCancellable on a real failure
- serialise setStar so overlapping toggles cannot clobber each other's rollback
- the menu star item reverts its label when the request fails (setStar now returns the
  result), so the open menu cannot drift from the row
- ContextMenuHost.dismiss clears the actions so a popped screen's ViewModel is released
- one shared setStar from the app shell instead of five per-ViewModel copies
- extract the nav-focus decision into focusPops and unit-test it

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

@@ -0,0 +1,39 @@
+package com.subtracks.ui
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class FocusPopsTest {
+    private val library = BackStackKey("library", null)
+    private val artistA = BackStackKey("artist/{artistId}?coverArt={coverArt}", "ar1")
+    private val albumA = BackStackKey("album/{albumId}?coverArt={coverArt}", "al1")
+    private val albumB = BackStackKey("album/{albumId}?coverArt={coverArt}", "al2")
+
+    @Test
+    fun anAbsentTargetNavigates() {
+        assertNull(focusPops(listOf(library, albumA), "artist/{artistId}?coverArt={coverArt}", "ar1"))
+    }
+
+    @Test
+    fun anEmptyStackNavigates() {
+        assertNull(focusPops(emptyList(), "album/{albumId}?coverArt={coverArt}", "al1"))
+    }
+
+    @Test
+    fun aTargetAlreadyOnTopNeedsNoPops() {
+        assertEquals(0, focusPops(listOf(library, artistA), "artist/{artistId}?coverArt={coverArt}", "ar1"))
+    }
+
+    @Test
+    fun aTargetUnderTheTopPopsDownToIt() {
+        assertEquals(1, focusPops(listOf(library, artistA, albumA), "artist/{artistId}?coverArt={coverArt}", "ar1"))
+    }
+
+    @Test
+    fun theMatchingEntryIsChosenNotJustTheDestination() {
+        val stack = listOf(library, albumA, artistA, albumB)
+        assertEquals(2, focusPops(stack, "album/{albumId}?coverArt={coverArt}", "al1"))
+        assertEquals(0, focusPops(stack, "album/{albumId}?coverArt={coverArt}", "al2"))
+    }
+}