Function klReduction

Quality benefit of upgrading current→next bits (port of _kl_reduction).

Two reference-mode semantics, autodetected by the sign of (kl_next - kl_cur):

• bf16 reference: sensitivities[b] = KL(bf16 || b-bit); upgrading reduces KL → benefit = kl_current - kl_next.
• uniform-N reference: sensitivities[baseline_bit] ≈ 0; sensitivities[b>base] measures how much upgrading moves output back toward bf16 (the benefit directly). Treat kl_next > kl_current as the uniform-mode signal.