Function fp64_pack_finite_or_inf 

pub fn fp64_pack_finite_or_inf(
    b: &CircuitBuilder,
    sign: Wire,
    mant_final_53: Wire,
    exp_after_round: Wire,
    stayed_sub_mask: Wire,
) -> Wire

Pack finite (normal / subnormal) and apply overflow-to-Inf if needed.

Input:

• sign (0/1), mant_final_53, exp_after_round
• stayed_sub_mask: all-1 iff we are in subnormal regime and there was no mantissa overflow

Output:

• finite_or_inf: packed 64-bit result (finite or +/−Inf on overflow)