Fix post processing for VIJHF datalayouts

The issue seems to be that `slab` for `VIJHF` returns `view(parent(data), :, :, h, :)`, which we try to index in `rmatmul1(M, in_slab, i, j)`, leading to scalar indexing. We may be able to simply wrap that call in an SArray, but I'd like to look at this more carefully.

Start by tracing the VIJHF datalayout path from slab into rmatmul1, focusing on the returned view and the reported scalar-indexing failure. Review how SArray is used in the surrounding post-processing code. Done means VIJHF post-processing no longer triggers scalar indexing, with the relevant behavior verified by the project's tests.

Written by the indexing model from the issue text.