The slot → on-disk element index map for the buffers currently on the GPU,
or undefined when none applies (identity: slot IS the on-disk index).
Type-checks the stored value so a malformed stamp reads as absent rather
than throwing on the hover path.
The slot → on-disk element index map for the buffers currently on the GPU, or
undefinedwhen none applies (identity: slot IS the on-disk index). Type-checks the stored value so a malformed stamp reads as absent rather than throwing on the hover path.