Record the slot → on-disk element index map describing the buffers this
commit just uploaded, or clear it when the commit produced none.
Called next to setCommittedData on every real commit — passing
undefined DELETES the key, so a previous commit's map can never outlive
the geometry it described (a stale map is a silently WRONG label, strictly
worse than falling back to the slot).
Record the slot → on-disk element index map describing the buffers this commit just uploaded, or clear it when the commit produced none.
Called next to setCommittedData on every real commit — passing
undefinedDELETES the key, so a previous commit's map can never outlive the geometry it described (a stale map is a silently WRONG label, strictly worse than falling back to the slot).