mixle.ppl._grid module

Structured-grid adjacency helper shared by field priors and the PDE solvers.

_grid_faces precomputes the fixed face list of a structured grid (every adjacent node pair along an axis, with 1/h^2 weights, plus the boundary mask). It is pure NumPy with no PDE-solver dependency, so it lives in mixle proper: the edge-preserving field priors (TotalVariation / Potts in mixle.ppl.priors) use it, and so does the PDE divergence-form assembly (now in the mixle-pde package, which imports it from here). Kept here so neither side depends on the other.