fix(#838): bound broker reply deadlines + fail-close empty-read; de-flake acceptance harness (#839)

Bound broker reply deadlines (separate read/lock/send budgets, BROKER_BUSY-before-mutation, fresh post-handle send budget → closes drop-after-commit window); fail-close empty/truncated read → GATE_UNAVAILABLE deny. De-flakes the 1917 acceptance surface. terra CODE APPROVE (pi) + Opus SECREV APPROVED (claude) — RoR comment 18143. Promote-lease-lost-ACK residual = fail-safe two-generals observability-gap, routed to WI-3 D2-v5 as named-disclosed-bounded-residual (route i). Gate-16 3-principal author=gpt-sol.

closes #838

packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts

@@ -1,5 +1,4 @@
 import { chmod, mkdir, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises';
-import { createConnection } from 'node:net';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { spawn, spawnSync, type ChildProcess } from 'node:child_process';
@@ -7,6 +6,7 @@ import { spawn, spawnSync, type ChildProcess } from 'node:child_process';
 import { afterEach, describe, expect, test } from 'vitest';
 
 import { launchClaudex, type ClaudexHarnessAdapter } from '../commands/claudex.js';
+import { requestBrokerReply } from '../lease-broker/broker-test-client.js';
 
 interface BrokerReply {
   ok: boolean;
@@ -43,17 +43,7 @@ const binding = (compaction_epoch = 1) => ({
 });
 
 async function request(socketPath: string, requestValue: object): Promise<BrokerReply> {
-  return await new Promise<BrokerReply>((resolve, reject) => {
-    const socket = createConnection(socketPath);
-    let response = '';
-    socket.setEncoding('utf8');
-    socket.once('error', reject);
-    socket.on('data', (chunk: string) => {
-      response += chunk;
-    });
-    socket.once('end', () => resolve(JSON.parse(response) as BrokerReply));
-    socket.once('connect', () => socket.end(`${JSON.stringify(requestValue)}\n`));
-  });
+  return await requestBrokerReply<BrokerReply>(socketPath, requestValue);
 }
 
 async function startBroker(): Promise<BrokerPaths> {