`FixedVectorShuffle` is a real two-input leading-block selection and
duplication network. Its closed `FixedVectorShuffleParams` record owns
integer-element widths, floating-element formats, positive maximum operand,
result, and block payload widths, and positive maximum source- and
result-block counts. Admission derives block geometry from the exact actor
types and requires both operands, the result, and every selector domain to fit
the concrete physical ports and these capacities.