feat: loop support via goto labels on each block

- Every block now emits a C label (its id) before its code
- generateNextCode emits goto label; for back-edges (visited targets)
- _emitBranch delegates to generateCode (not generateOwnCode), so labels
  are emitted consistently in both main and branch traversals
- generateCallWithError threads a shared visited set across all three
  emitBranch calls, preventing duplicate label emission for shared blocks
- emitBranch skips roots already visited by previous roots in the same
  branch (prevents spurious gotos from shared-block traversal)

codegen/index.js

@@ -46,8 +46,9 @@
 
   /**
    * Emits C code for a branch by walking from root blocks, stopping at
-   * blocks in the stop set and at decision boundaries.  Each visited block
-   * is added to the visited set to prevent double-emission.
+   * blocks in the stop set and at decision boundaries.  When a loop
+   * back-edge encounters an already-visited block, a `goto label;` is
+   * emitted instead of silently dropping the edge.
    * @param {Array} roots
    * @param {Set} stop  blocks to exclude (shared between branches)
    * @param {Set} visited  blocks already emitted
@@ -60,24 +61,20 @@
       function (b) {
         return b.isDecision;
       };
-    var v = new Set(visited);
     var parts = [];
 
     function walk(el) {
-      if (v.has(el) || (stop && stop.has(el))) return;
-      v.add(el);
-      if (isDecision(el)) {
-        parts.push(el.generateCode(v));
-      } else {
-        parts.push(el.generateOwnCode());
-        var flow = el.wcNextFlow || el.nextFlow || [];
-        for (var j = 0; j < flow.length; j++) {
-          walk(flow[j].toElement);
-        }
+      if (stop && stop.has(el)) return;
+      if (visited.has(el)) {
+        parts.push("\n  goto " + (el.id || "block") + ";\n");
+        return;
       }
+      parts.push(el.generateCode(visited));
     }
 
-    for (var i = 0; i < roots.length; i++) walk(roots[i]);
+    for (var i = 0; i < roots.length; i++) {
+      if (!visited.has(roots[i])) walk(roots[i]);
+    }
     return parts.join("");
   };
 
@@ -104,12 +101,15 @@
       if (okAll.has(b)) shared.add(b);
     });
 
-    var errCode = CodeGen.emitBranch(errTargets, shared, new Set(), isDecision);
-    var okCode = CodeGen.emitBranch(okTargets, shared, new Set(), isDecision);
+    // Thread a shared visited set through all three emitBranch calls
+    // so shared blocks are not re-emitted with duplicate labels.
+    var visited = new Set();
+    var errCode = CodeGen.emitBranch(errTargets, shared, visited, isDecision);
+    var okCode = CodeGen.emitBranch(okTargets, shared, visited, isDecision);
     var sharedCode = CodeGen.emitBranch(
       Array.from(shared),
       new Set(),
-      new Set(),
+      visited,
       isDecision
     );