Pixal3D refactored the container into a VarLenTensor base (feats + explicit
slice-per-batch layout, no coords) with SparseTensor(VarLenTensor) adding
coordinates. LATO.2 has only the combined class. Modelling the split here means one
package serves both without either port adapting at every call site.
Also exposes Pixal3D's cache spelling (get/register_spatial_cache) alongside LATO.2's
(cache_get/cache_put). 13/13 tests unchanged.
Extracted from lato.2_mrp_mlx. The same sparse module underlies LATO.2, Pixal3D and
the rest of the TRELLIS.2 family, and all of them are blocked on Apple Silicon by the
same single op - submanifold conv - so it belongs in one tested package rather than
vendored per port.
13/13 tests: 7 for the conv against a hand-written reference (spconv is uninstallable
here so there is no upstream oracle), 6 for the remaining layers against torch.
SubMConv3d runs 13.8ms at 128^3/128ch on m3ultra, 5.9x faster than the obvious
per-offset loop.
