Function fp64_mul_finish_specials 

pub fn fp64_mul_finish_specials(
    b: &CircuitBuilder,
    pa: &Fp64Parts,
    pb: &Fp64Parts,
    sign_xor: Wire,
    finite_result: Wire,
) -> Wire

[Block M3] Apply multiplication-specific specials overlay.

Rules (precedence):

1. NaN if any NaN, or (Inf * 0) either order → canonical quiet NaN
2. Else if any Inf → return ±Inf with XOR sign
3. Else if any Zero → return signed zero with XOR sign
4. Else → use the finite pipeline result

Parameters

• pa, pb: Parts from fp64_unpack for operands a and b
• sign_msb: MSB boolean of XOR of input signs
• finite_result: Result from finite multiplication pipeline

Returns

• Final 64-bit IEEE-754 result with special cases handled