Function classify_equality_rule 

pub fn classify_equality_rule(
    l: &Node,
    r: &Node,
    op: &str,
    env: &Env,
) -> &'static str

Equality-layer classification used by both build_proof and the per-query provenance walker. Precedence (issue #97): assigned > structural > definitional > numeric. Returns the rule string verbatim so JS and Rust emit identical labels.