d guard (carried)

The XVI-P p23 dred is imported unchanged, with one added rule. Polynomial construction (Poly.new, a class attribute) and any internal rename of $d$ are admitted only when the entry sympy function called from user code is an allow-listed rational normaliser; otherwise they are denied. This closes the p23 bypasses: sympy.polys.rem, pre-import capture, reduced, groebner, the Dummy rename and rep.eval. Probe: 16 forms $\times$ 4 expressions all raise, and ordinary algebra is unchanged.