gam-model-kernels — the shared BASE kernel layer carved out of
gam-models under #1521. These are the family-agnostic infrastructure
modules that every concrete family (survival / bms / gamlss / multinomial /
custom) consumes: the cubic-cell / cell-moment kernels, the design-scaling,
link, and root-solving primitives.
The crate sits strictly BELOW gam-models: it depends only on the lower
tiers (gam-linalg, gam-math, gam-problem, gam-runtime, gam-solve)
and on its own sibling modules — never up into a concrete family. The
modules are re-exported at the gam-models crate root so the relocated
families' crate::cubic_cell_kernel::* / crate::scale_design::* paths
resolve unchanged.