iOS: Random Album Radio plays whole albums in random order, loading each next album lazily

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ios/Stemify/Playback/PlaybackManager.swift

@@ -201,7 +201,41 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
         messageListeners[id] = f
         return id
     }
-    private func changed() { listeners.values.forEach { $0() } }
+    private func changed() {
+        listeners.values.forEach { $0() }
+        topUpRadio()
+    }
+
+    /// Album radio: every album in the library in random order, each played whole and in track order.
+    /// Only the next album is fetched, once the queue is nearly played out.
+    @ObservationIgnored private var radioDeck: [Album]?
+    @ObservationIgnored private var radioBusy = false
+
+    func startAlbumRadio() async throws {
+        var deck = try await plex.albums().shuffled()
+        var first: [Track] = []
+        while first.isEmpty, !deck.isEmpty { first = (try? await plex.albumTracks(deck.removeLast().ratingKey)) ?? [] }
+        if first.isEmpty { return }
+        playTracks(first)
+        radioDeck = deck
+    }
+
+    private func topUpRadio() {
+        guard radioDeck != nil, !radioBusy, queue.count - index < 3 else { return }
+        radioBusy = true
+        Task { @MainActor in
+            defer { radioBusy = false }
+            while var deck = radioDeck, !deck.isEmpty {
+                let album = deck.removeLast()
+                radioDeck = deck
+                if let t = try? await plex.albumTracks(album.ratingKey), !t.isEmpty, radioDeck != nil {
+                    addToQueue(t)
+                    return
+                }
+            }
+            if radioDeck != nil, let all = try? await plex.albums(), !all.isEmpty { radioDeck = all.shuffled() }
+        }
+    }
 
     func positionMs() -> Int64 {
         let snap = engine.snapshot
@@ -229,6 +263,7 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
 
     func playTracks(_ tracks: [Track], mixes: [[String: StemSetting]]? = nil, start: Int = 0, shuffle doShuffle: Bool = false) {
         if tracks.isEmpty { return }
+        radioDeck = nil
         let items = items(tracks, mixes)
         if doShuffle {
             unshuffled = items