ios: prepare the next track only after the current one is playing and fully cached

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

ios/Stemify/Playback/PlaybackManager.swift

@@ -654,7 +654,11 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
         guard let i = upcomingIndex() else { return }
         let item = queue[i]
         nextTask = Task { @MainActor [weak self] in
-            try? await Task.sleep(nanoseconds: 4_000_000_000) // let the current track's stems get a head start
+            // Don't compete with the current track: wait until it is playing and fully cached.
+            while let self, !Task.isCancelled {
+                if let cur = self.current?.track, self.engine.playing, !self.engine.buffering, self.playableOffline(cur) { break }
+                try? await Task.sleep(nanoseconds: 1_000_000_000)
+            }
             guard let self, !Task.isCancelled else { return }
             // The normal load path reports failures.
             guard let (s, entry) = try? await self.openDetached(item.track, 0) else { return }