Completes the sparse op surface Pixal3D's decoders need.
spatial2channel/channel2spatial are sparse space-to-depth and its inverse. The slot
index is sum_i (coord[i] % f) * f**i - axis 0 FASTEST varying, the reverse of C order.
decodes it the same way; using C order in one and this in the other would
misplace every child while keeping all shapes valid.
SparseResBlockC2S3d widens to out_channels*8 so channel2spatial can redistribute those
channels across the 8 children, with a predicted subdiv mask deciding which children
exist - so the occupied set grows selectively (32 voxels -> 122, not 256).
Tests cover exact round-trip, zero-fill of unoccupied slots, and selective growth.
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.
