Function fp64_underflow_shift 

pub fn fp64_underflow_shift(
    b: &CircuitBuilder,
    res_sig: Wire,
    res_exp: Wire,
) -> (Wire, Wire, Wire)

Pre-round underflow handling: if exp<=0, right shift by k=1-exp and fold sticky into bit0. Also prepare the exponent for rounding geometry (Exp=1).

Input: (res_sig, res_exp)

Output:

• (sig_round_base, exp_round_base, exp_lt_1_mask) where exp_round_base = (exp<=0 ? 1 : exp).