Function fp64_round_rne 

pub fn fp64_round_rne(
    b: &CircuitBuilder,
    sig_base: Wire,
    exp_base: Wire,
) -> (Wire, Wire, Wire)

Round-to-nearest, ties-to-even (RN-even).

Geometry:

• Integer bit at 63; we interpret bits:
  • LSB of target mantissa at bit 11
  • Guard=10, Round=9, Sticky=bit 0 (already folded)

Input: (sig_base, exp_base)

Output:

• (mant_final_53, exp_after_round, mant_overflow_mask)
  • mant_final_53 is a 53-bit value (includes hidden 1 for normals)
  • If mant overflowed to 54 bits, we shift right 1 and increment exponent.