Sealed IBinPolyMul impl selected (alongside Medium) on the scalar backend when PCLMUL is not available (net461 / netstandard2.0, or netcoreapp3+ with PCLMULQDQ disabled). Picked for sizes at or above KaratsubaCutoff; smaller sizes route to Medium instead. Implements multiplication via an in-class Karatsuba recursion whose leaf is the scalar 16-entry-table ImplMul. The class name reflects the dispatch criterion (large operand size); the algorithm inside is Karatsuba.