Slab construction

surface_basis(hkl) performs Euclid's algorithm on the three Miller indices as column operations on the identity. The result is an integer, right-handed, unimodular basis whose first two rows lie in (hkl) and whose third row satisfies h . v3 = 1.

The slab lattice is [a, b, normal * sign(rise) * height], where height = layers * |rise| + vacuum. Atoms are placed in surface coordinates: one layer is one unit along the third basis vector, the in-plane component of c is removed by solving it in the a,b plane, and the vacuum becomes the final fractional range. SLAB_FLOOR keeps the slab off the cell face so atoms on the face are not drawn twice.

The cut is P1. Bond carrying through the cut needs the bond graph of Task 1.11 and is not in this pass; Slab.cut is zero until then.