Constant SIGNATURE_PROXIMITY_K 

pub const SIGNATURE_PROXIMITY_K: u32 = 1;

The signature-shape proximity threshold — 2-adic valuation ≥ k bits of (digest XOR reference) for admission. The shipped k = 1 yields accept_prob = 1/2 (digest’s LSB matches the reference’s LSB); larger k tightens the signature-shape admission predicate per ADR-049’s bandwidth-additivity.