Sealed IBinPolyMul impl selected for sizes at or above the PCLMUL KaratsubaCutoff. Implements multiplication via a Karatsuba recursion that bottoms out via ImplLeaf into the uniform ImplMulEven / ImplMulOdd arbitrary-degree-Karatsuba leaf (by even/odd length). The class name reflects the dispatch criterion (large operand size); the algorithm inside is Karatsuba.