fix(rectification): surface dropped probes and filter tools by the decision

Silent unrenderable discriminators, a missing question-contract golden, and a always-on tool table were hiding fail-closed drops behind the prompt wall.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/tests/rectification-v9-engine-contract.test.ts

@@ -261,6 +261,25 @@ test("runV9CandidateScore strictly consumes server candidate decisions and v2 re
   }
 });
 
+test("runV9CandidateScore fails closed on a mismatched question contract version", async () => {
+  const restore = stubEngine({
+    ...ENGINE_SCORE_RESPONSE_V2,
+    decision_receipt: { ...DECISION_RECEIPT, question_contract_version: "probe-question-v0" },
+  });
+  try {
+    await assert.rejects(
+      runV9CandidateScore({
+        baselineBirthSnapshot: SNAPSHOT,
+        candidateRange: RANGE,
+        events: toEngineEvents(EVIDENCE),
+      }),
+      (error: unknown) => error instanceof RectificationEngineError && error.code === "engine_invalid_v2_receipt",
+    );
+  } finally {
+    restore();
+  }
+});
+
 test("runV9CandidateScore fails closed without a v2 decision receipt", async () => {
   const missingReceipt = Object.fromEntries(
     Object.entries(ENGINE_SCORE_RESPONSE_V2).filter(([key]) => key !== "decision_receipt"),