Fox--Neuwirth model for planar configurations

The public API contains the barred-permutation face combinatorics, prime shuffle arithmetic, the concrete compact top-cell polyhedron, its free prime-symmetry action, the equivariant configuration and reference maps, the full finite face poset, canonical cell orientations, signed codimension-one boundary relations, the order-complex skeleton, modulo-prime orbit cancellation, and the reference-zero orbit count.