Shared SliceCache helpers for the three progressive loaders (GSplats, Points,
Lines). Keeps the per-slice key, snapshot cloning, and lookup/store logic
identical across geometries so the loaders stay symmetric and can't drift.
Also used by the three PLAIN spatial-index loaders (plain-leaf nodes, no
additive ladder): a plain leaf caches its decoded slice as a 1-element
ladder under the same key contract, so plain and progressive nodes share
one cache, one signature, and one clone/budget policy.
Shared SliceCache helpers for the three progressive loaders (GSplats, Points, Lines). Keeps the per-slice key, snapshot cloning, and lookup/store logic identical across geometries so the loaders stay symmetric and can't drift.
Also used by the three PLAIN spatial-index loaders (plain-leaf nodes, no additive ladder): a plain leaf caches its decoded slice as a 1-element ladder under the same key contract, so plain and progressive nodes share one cache, one signature, and one clone/budget policy.