Add SWEL (self-dual with equivalent logicals) support for CSS codes over arbitrary finite fields

[perlinm]

AI-assisted summary

A SWEL code is a self-dual CSS code admitting a logical-operator basis with Lx = Lz, which is what enables transversal logical Hadamard and S gates.

New:

• CSSCode.is_swel: basis-independent test for whether such a basis exists. Supports qubits, qudits over GF(q), and subsystem codes, with cheap short-circuits from arXiv:2503.19790.
• CSSCode.get_swel_logical_ops / CSSCode.set_swel_logical_ops: construct / install such a basis, or raise if none exists.
• math.get_orthonormal_basis: generic routine returning an orthonormal basis (L @ L.T = I) for the row space of a matrix over any finite field, or None if none exists; handles rank-deficient input (with a promise_full_rank fast path).

Also in this PR:

• In-place setters (set_logical_ops*, forget_distance, reduce_logical_ops, ...) return self.
• Code transformations (maybe_to_css, canonicalized, conjugated, ...) preserve cached properties such as dimension and distance.
• Use galois row_space() in place of row_reduce() + zero-row removal where a row-space basis is intended (common.py, rings.py, encoding.py).