Method add_equiv

crates/kernel/src/equiv.rs:132–136

Record that two composite keys are definitionally equal. Kept by-value because `node_for_key` inserts the key into the internal `key_to_node` map on first observation, requiring ownership transfer. Callers that have already consumed their `EqKey`s should clone at the call site, not here.

(&mut self, k1: EqKey, k2: EqKey)

    true
  }

  /// Check if two composite keys are equivalent.
  ///
  /// Takes keys by reference — callers in the `is_def_eq` hot path
  /// already hold `EqKey` tuples as local bindings, and forcing them to
  /// pass by value would require an Arc-clone on each component. With
  /// by-ref we avoid that clone entirely (see `src/ix/kernel/def_eq.rs`
  /// for the caller pattern).