feat: add TrackContextMenuConfig value type

Pure value type bundling playlists and callbacks needed to build a track
context menu; both tests pass (TDD).

Music/Models/TrackContextMenuConfig.swift

@@ -0,0 +1,10 @@
+import Foundation
+
+struct TrackContextMenuConfig {
+    let playlists: [Playlist]
+    let lastUsedPlaylistName: String?
+    let selectedPlaylist: Playlist?
+    let onAddToPlaylist: (Track, Playlist) -> Void
+    let onAddToLastPlaylist: ((Track) -> Void)?
+    let onRemoveFromPlaylist: ((Track) -> Void)?
+}

MusicTests/TrackContextMenuConfigTests.swift

@@ -0,0 +1,72 @@
+import Testing
+@testable import Music
+
+struct TrackContextMenuConfigTests {
+    // Builds a config with all fields set and verifies:
+    // - stored playlists, lastUsedPlaylistName, selectedPlaylist match the inputs
+    // - onAddToPlaylist callback fires with the correct track and playlist
+    // - onAddToLastPlaylist callback fires with the correct track
+    // - onRemoveFromPlaylist callback fires with the correct track
+    // - when optional callbacks are nil, optionally calling them is safe
+
+    @Test func storesPropertiesAndFiresCallbacks() {
+        // 1. Create fixture data
+        let pl1 = Playlist.fixture(id: 1, name: "Favorites")
+        let pl2 = Playlist.fixture(id: 2, name: "Chill")
+        let track = Track.fixture(id: 42, title: "Test")
+
+        var addedTrack: Track? = nil
+        var addedPlaylist: Playlist? = nil
+        var lastTrack: Track? = nil
+        var removedTrack: Track? = nil
+
+        // 2. Build config with all callbacks
+        let config = TrackContextMenuConfig(
+            playlists: [pl1, pl2],
+            lastUsedPlaylistName: "Favorites",
+            selectedPlaylist: pl1,
+            onAddToPlaylist: { t, p in addedTrack = t; addedPlaylist = p },
+            onAddToLastPlaylist: { t in lastTrack = t },
+            onRemoveFromPlaylist: { t in removedTrack = t }
+        )
+
+        // 3. Verify stored properties
+        #expect(config.playlists.count == 2)
+        #expect(config.playlists[0].name == "Favorites")
+        #expect(config.lastUsedPlaylistName == "Favorites")
+        #expect(config.selectedPlaylist == pl1)
+
+        // 4. Invoke callbacks and verify they fire correctly
+        config.onAddToPlaylist(track, pl2)
+        config.onAddToLastPlaylist?(track)
+        config.onRemoveFromPlaylist?(track)
+
+        #expect(addedTrack?.id == track.id)
+        #expect(addedPlaylist?.id == pl2.id)
+        #expect(lastTrack?.id == track.id)
+        #expect(removedTrack?.id == track.id)
+    }
+
+    @Test func nilOptionalCallbacksAreSafe() {
+        // Verifies that a config with nil optional callbacks does not crash
+        // when you call them via optional chaining (the normal usage pattern)
+        let pl = Playlist.fixture(id: 1, name: "Rock")
+        let track = Track.fixture()
+
+        let config = TrackContextMenuConfig(
+            playlists: [pl],
+            lastUsedPlaylistName: nil,
+            selectedPlaylist: nil,
+            onAddToPlaylist: { _, _ in },
+            onAddToLastPlaylist: nil,
+            onRemoveFromPlaylist: nil
+        )
+
+        // These must not crash
+        config.onAddToLastPlaylist?(track)
+        config.onRemoveFromPlaylist?(track)
+
+        #expect(config.lastUsedPlaylistName == nil)
+        #expect(config.selectedPlaylist == nil)
+    }
+}