Function head_const_name

crates/kernel/src/def_eq.rs:1661–1664

Extract head constant's display form as a string, for diagnostic prefix matching. Uses `{kid}`'s Display impl (which is defined for every `KernelMode`), not the inner `Name` which only has Display in Meta mode. Returns `None` if the head isn't a `Const`.

(e: &KExpr<M>)

    }
    log::info!(
      "[eta] const={} depth={} reason={} id={} idx={} a={} b={}",
      self.debug_label.as_deref().unwrap_or("<unknown>"),
      self.def_eq_depth,
      reason,
      id_s,
      idx,
      compact_def_eq_expr(a),