feat(graph): add structuralActionRequired contract (57J.67)

- Add structuralActionRequired field to graphUpdateSchema (optional boolean nullable)
- Validate declaration consistency in validateGraphUpdate():
  - true requires meaningful mutation (addedNodes/updatedNodes/addedEdges)
  - false permits intentional no-op when userSupportedMeaning populated
  - null/absent with meaning → reject
  - true/false mismatch on output shape → reject
  - preserve legacy no-op guard for non-contract paths
- Update prompt-builder: add field to required list, insert contract section between rules and Additional Guidance with two mandatory sentences
- 50 new tests: schema validation (4), prompt builder content checks (10), utils contract matrix (10), plus 26 existing suite migrations

All 197 graph tests pass.

lib/graph/utils.js

@@ -865,7 +865,9 @@ export function validateGraphUpdate(graph, update) {
     }
   }
 
-  // Reject updates with no meaningful change
+  // ── structuralActionRequired contract (57J.67) ───────────
+
+  const meaningPopulated = !!update.answerMeaning?.userSupportedMeaning;
   const statusChanged = update.updatedNodes.some(
     (u) => u.previousStatus !== null && u.newStatus !== u.previousStatus,
   );
@@ -880,14 +882,38 @@ export function validateGraphUpdate(graph, update) {
     update.addedEdges.length > 0 ||
     update.removedEdgeIds.length > 0;
 
-  if (!hasMeaningfulChange) {
-    // Specific diagnostic for the semantic-only no-op case: populated userSupportedMeaning with zero structural mutation.
-    // The validator does NOT determine whether meaning is "new" or "consequential" — it only observes that meaning exists without structural expression.
-    if (update.answerMeaning?.userSupportedMeaning) {
-      errors.push(
-        "answerMeaning.userSupportedMeaning is populated, but the proposal contains no graph mutation. answerMeaning alone does not constitute graph progress.",
-      );
-    } else {
+  // Missing/null transition rule: must be present when userSupportedMeaning is populated
+  if (
+    (update.structuralActionRequired === null || update.structuralActionRequired === undefined) &&
+    meaningPopulated
+  ) {
+    errors.push(
+      "structuralActionRequired must be present when userSupportedMeaning is populated",
+    );
+  }
+
+  // Exact structural claim — four contradiction pairs
+  if (update.structuralActionRequired === true && !hasMeaningfulChange) {
+    errors.push("structuralActionRequired is true but proposal contains no graph mutation");
+  }
+  if (update.structuralActionRequired === false && hasMeaningfulChange) {
+    errors.push("structuralActionRequired is false but proposal contains meaningful mutations");
+  }
+
+  // Legacy no-op guard: only fires when structuralActionRequired !== false.
+  // When false → zero-mutation is a valid intentional no-op (contract PASS).
+  if (!hasMeaningfulChange && update.structuralActionRequired !== false) {
+    if (meaningPopulated) {
+      // Avoid double-error when field absence was already flagged above
+      if (
+        update.structuralActionRequired !== null &&
+        update.structuralActionRequired !== undefined
+      ) {
+        errors.push(
+          "answerMeaning.userSupportedMeaning is populated, but the proposal contains no graph mutation. answerMeaning alone does not constitute graph progress.",
+        );
+      }
+    } else if (!meaningPopulated) {
       errors.push("Update contains no meaningful change");
     }
   }