Supercells

The new lattice is P M for a positive integer matrix P. A fractional point x of the old cell appears in the new cell at (x + n) P^-1; the code enumerates the integer shifts in a box bounded by 4 ||P^-1||_max + 2, keeps the images in [0, 1) and drops boundary duplicates through the same fractional grid used by the neighbor search (Task 1.10).

The atom count is n_atoms(P1) |det P|: a structure with symmetry is expanded to P1 before the repeat, so a supercell of a 2-atom silicon cell with 2 x 2 x 2 has 16 atoms and not 2. The result is P1: the old group does not apply in the new setting and is re-detected later. The size check from Task 1.3 runs before allocation and a hard-limit verdict becomes Error::LimitExceeded.

origin_shift adds a fractional vector and rewraps the sites; the lattice and metadata are unchanged.