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

Music/Models/EditableTrackFields.swift

@@ -49,7 +49,9 @@ nonisolated struct EditableTrackFields: Equatable, Sendable {
 
     // Returns prefill values (from the first track) plus the set of fields whose
     // values are NOT identical across all tracks (shown as "Mixed" in the UI).
+    // Precondition: caller must pass at least one track; an empty array will trap.
     static func shared(across tracks: [Track]) -> (values: EditableTrackFields, mixed: Set<EditableTrackField>) {
+        precondition(!tracks.isEmpty, "shared(across:) requires at least one track")
         let base = EditableTrackFields(from: tracks[0])
         var mixed: Set<EditableTrackField> = []
         for t in tracks.dropFirst() {

MusicTests/EditableTrackFieldsTests.swift

@@ -48,4 +48,20 @@ struct EditableTrackFieldsTests {
         let f = EditableTrackFields(from: t)
         #expect(f.apply(editing: [], to: t) == t)
     }
+
+    @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))
+    }
 }