syncTo

Points this index at cues, reusing what it already computed when it can.

The engine's cue table grows by appending decoded batches, which is the hot path and is extended in place. It is also merged when a decoder emits out of order, pruned from the front as playback moves on, and cleared on a track change, and each of those invalidates every position. The identity check on the last cue this index was built from is what tells the two apart: after a pure append it still sits where it did, and after anything else it does not.