fix: tighten stack-depth guard to !== 4 to catch overflow (#584)

Reviewer noted that `< 4` only catches underflow; programs leaving 5+
values on the DYADIC stack silently passed isValid().  Change the guard
to `!== 4` so both under- and overflow are rejected, matching the
documented 'exactly 4 outputs' contract.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/push3-transpiler/src/transpiler.ts

@@ -391,7 +391,7 @@ export function transpile(program: Node): TranspileResult {
   processItems(program.items, state);
 
   // Pop 4 outputs: top → ci, then anchorShare, anchorWidth, discoveryDepth.
-  if (state.dStack.length < 4) {
+  if (state.dStack.length !== 4) {
     throw new Error(
       `Program must leave exactly 4 values on the DYADIC stack; found ${state.dStack.length}`,
     );