Redis-backed Event Bus broker (feature-flagged via REDIS_URL)

Closes gap-analysis v2 §7.3 and §10.3.

- New services/eventBusBroker.ts: per-process singleton with unique
  instanceId; subscribes to a Redis pub/sub channel (default
  ccombo:events) and re-emits inbound records on the in-process
  EventEmitter. Outbound envelopes carry the instanceId so the
  originating replica drops its own echo.
- eventBus.publish() hands every persisted record to the broker after
  the local emit, so same-process SSE subscribers see zero change
  while remote-replica subscribers now receive every event.
- When REDIS_URL is unset the broker is strictly inert (no connect,
  no publish) — single-process behaviour is preserved byte-for-byte.
- 3 unit tests for feature-flag semantics; full suite 83/83 green.
- Live Redis behaviour will be covered by the Testcontainers suite
  in PR Q.

orchestrator/tests/unit/eventBusBroker.test.ts

@@ -0,0 +1,64 @@
+/**
+ * Tests for the Redis-backed cross-replica broker (gap v2 §7.3 / §10.3).
+ *
+ * These tests exercise the broker's feature-flag semantics without
+ * spinning up a real Redis: when `REDIS_URL` is unset the broker must
+ * be strictly inert. Live Redis behaviour is covered in the integration
+ * suite (PR Q).
+ */
+
+import { EventEmitter } from "events";
+import { getBroker, __resetBrokerForTests } from "../../src/services/eventBusBroker";
+import type { EventRecord } from "../../src/services/eventBus";
+
+describe("eventBusBroker (feature flag)", () => {
+  const savedEnv = { ...process.env };
+
+  beforeEach(() => {
+    process.env = { ...savedEnv };
+    __resetBrokerForTests();
+  });
+
+  afterEach(() => {
+    __resetBrokerForTests();
+    process.env = { ...savedEnv };
+  });
+
+  it("is inert when REDIS_URL is unset", async () => {
+    delete process.env.REDIS_URL;
+    const emitter = new EventEmitter();
+    const broker = getBroker(emitter);
+    expect(broker.enabled).toBe(false);
+    expect(typeof broker.instanceId).toBe("string");
+    const rec: EventRecord = {
+      id: "00000000-0000-0000-0000-000000000001",
+      plan_id: "11111111-1111-1111-1111-111111111111",
+      type: "transaction.created",
+      actor: null,
+      payload: {},
+      payload_hash: "h",
+      prev_hash: null,
+      signature: "s",
+      created_at: new Date().toISOString(),
+    };
+    await expect(broker.publish(rec)).resolves.toBeUndefined();
+    await expect(broker.close()).resolves.toBeUndefined();
+  });
+
+  it("returns a stable singleton within a process", () => {
+    delete process.env.REDIS_URL;
+    const emitter = new EventEmitter();
+    const a = getBroker(emitter);
+    const b = getBroker(emitter);
+    expect(a).toBe(b);
+  });
+
+  it("generates a unique instanceId per process", () => {
+    delete process.env.REDIS_URL;
+    const emitter = new EventEmitter();
+    const a = getBroker(emitter).instanceId;
+    __resetBrokerForTests();
+    const b = getBroker(emitter).instanceId;
+    expect(a).not.toEqual(b);
+  });
+});