Persisting and restoring the playback position

Saving "where the user was" has two very different costs. Rewriting the whole queue means a full list serialized to storage; saving the position means two small values. Doing the expensive one on a timer is wasteful, and doing the cheap one only on lifecycle edges misses the case that matters most: a long uninterrupted background session ended by the system, with no pause and no track change to trigger a save.

So there are two save shapes with different triggers: