perf(core): applySubst skips empty subst and shares unchanged subtrees (fib 250->187ms, 270/270)

Author: MesTTo

packages/core/src/substitution.ts

@@ -25,11 +25,20 @@ export function extendSubst(s: Subst, x: string, a: Atom): Subst {
 /** Apply `s` to an atom: replace each variable by its assigned value (one pass; the substituted
  *  value is not itself re-substituted). */
 export function applySubst(s: Subst, a: Atom): Atom {
+  if (s.length === 0) return a; // empty substitution is identity (no clone)
   switch (a.kind) {
     case "var":
       return lookupSubst(s, a.name) ?? a;
-    case "expr":
-      return { ...a, items: a.items.map((it) => applySubst(s, it)) };
+    case "expr": {
+      // Clone only if a child actually changed; otherwise share the reference (no allocation).
+      let changed = false;
+      const items = a.items.map((it) => {
+        const r = applySubst(s, it);
+        if (r !== it) changed = true;
+        return r;
+      });
+      return changed ? { ...a, items } : a;
+    }
     default:
       return a;
   }