harden: precondition + multi-track test for EditableTrackFields.shared

MusicTests/EditableTrackFieldsTests.swift

@@ -33,6 +33,22 @@ struct EditableTrackFieldsTests {
         #expect(!mixed.contains(.artist))
     }
 
+    @Test func sharedAcrossThreeTracksAccumulatesMixed() {
+        // Step 1: three tracks all share the same album, but title differs on the
+        //         third track and genre differs on the second — so both title and
+        //         genre must end up "mixed", while album stays shared.
+        let t1 = Track.fixture(title: "Same", album: "One Album", genre: "Rock")
+        let t2 = Track.fixture(title: "Same", album: "One Album", genre: "Pop")
+        let t3 = Track.fixture(title: "Different", album: "One Album", genre: "Rock")
+        // Step 2: shared() over all three.
+        let (values, mixed) = EditableTrackFields.shared(across: [t1, t2, t3])
+        // Step 3: album is shared (not mixed); title + genre are mixed.
+        #expect(values.album == "One Album")
+        #expect(!mixed.contains(.album))
+        #expect(mixed.contains(.title))
+        #expect(mixed.contains(.genre))
+    }
+
     @Test func applyOnlyWritesEditedFields() {
         // Step 1: a track and a fields object that changes album only.
         let t = Track.fixture(album: "Old", genre: "Rock")