feat(02-04): harness A25 — empirical <5s SAVE→zip latency (REQ-archive-export-latency, SPEC §10 #6)

Wire A25 into the UAT harness as the binding empirical gate for
REQ-archive-export-latency / SPEC §10 #6 (5000ms hard ceiling end-to-end
from SAVE_ARCHIVE dispatch to zip-on-disk).

Architecture:
- Page-side assertA25 records t0 (performance.now) + t0Wall (Date.now)
  + tAck bookends around the chrome.runtime.sendMessage(SAVE_ARCHIVE)
  call. Returns A25Result extending AssertionRecord with the 3 timing
  fields + ackSuccess flag.
- Host-side driveA25(page, downloadsDir) snapshots zip dir BEFORE
  page.evaluate dispatch, polls for new-or-overwritten .zip via mtime
  delta (mirrors A12/A13 overwrite-aware pattern), uses page-supplied
  t0Wall as the host anchor for the dispatch→file-on-disk latency
  check (NOT a host-side Date.now captured before page.evaluate, which
  would include setupFreshRecording + 11s segment-settle wall time and
  always fail the 5s budget).

[Rule 1 - Bug] Initial implementation used host-side Date.now() captured
before page.evaluate as the latency anchor — this incorrectly included
the 11s segment-settle window in the budget. First run observed
A25.3=11188ms (FAIL). Fix: page-side captures Date.now() at the
SAVE_ARCHIVE dispatch instant (AFTER setupFreshRecording + segment-settle
complete) and returns it as t0Wall in A25Result; the driver uses this
as the canonical host anchor. Result on re-run: A25.3=61ms (GREEN, well
under 5s SLO). Documented per T-02-04-02 disposition (bracket only the
SAVE dispatch, not the broader test orchestration).

Files modified:
- tests/uat/extension-page-harness.ts (+~115 lines): assertA25 +
  A25_* constants + A25Result interface
- tests/uat/lib/harness-page-driver.ts (+~95 lines): driveA25 +
  A25_HOST_POLL_TIMEOUT_MS const + A25_LATENCY_CEILING_MS const
- tests/uat/harness.test.ts (+~15 lines): import driveA25, wrap with
  downloadsDir, append to drivers list

Verification:
- HEADLESS=1 npm run test:uat → 26/26 GREEN
- elapsedAck=60ms, host-side delta=61ms (both well under 5000ms SLO)
- npx vitest run tests/background/no-test-hooks-in-prod-bundle.test.ts
  → 13/13 GREEN (Tier-1 FORBIDDEN_HOOK_STRINGS unchanged at 12)
- npx tsc --noEmit → clean

Plan 02-04 scope: 2/3 tasks landed (A24 + A25); Task 3 adds
A26 (meta.json 8-field) + A27 (multi-tab strict) + A28 (archive-layout strict).

tests/uat/harness.test.ts

@@ -91,6 +91,8 @@ import {
   driveA23,
   // Plan 02-04 Task 1 — D-P2-01 empirical Blob URL verification
   driveA24,
+  // Plan 02-04 Task 2 — REQ-archive-export-latency (5s ceiling)
+  driveA25,
   getManifestVersion,
 } from './lib/harness-page-driver';
 import {
@@ -259,7 +261,7 @@ async function assertA0_GrepGate(): Promise<{
  */
 async function main(): Promise<number> {
   process.stdout.write('\nMokosh Plan 01-13 + 01-14 + 02-04 — UAT harness orchestrator\n');
-  process.stdout.write('Architecture: A0 pre-flight + extension-internal page driver (A1..A14, A15..A17, A18..A22, A23, A24)\n');
+  process.stdout.write('Architecture: A0 pre-flight + extension-internal page driver (A1..A14, A15..A17, A18..A22, A23, A24, A25)\n');
   process.stdout.write('='.repeat(72) + '\n');
 
   // A0 pre-flight (no Chrome launch needed; runs against built dist/).
@@ -313,6 +315,10 @@ async function main(): Promise<number> {
     (page) => driveA12(page, handles.downloadsDir);
   const driveA13Wrapped: (page: import('puppeteer').Page) => Promise<AssertionRecord> =
     (page) => driveA13(page, handles.downloadsDir, expectedManifestVersion);
+  // Plan 02-04 Task 2 — driveA25 needs downloadsDir for the host-side
+  // dispatch→file-on-disk latency check (mirrors A5/A12/A13 wrapping).
+  const driveA25Wrapped: (page: import('puppeteer').Page) => Promise<AssertionRecord> =
+    (page) => driveA25(page, handles.downloadsDir);
 
   const drivers: ReadonlyArray<{
     readonly name: string;
@@ -384,6 +390,14 @@ async function main(): Promise<number> {
     // must be installed pre-dispatch. After A24 the recording stays alive
     // for any chained Plan 02-04 Tasks 2-3 assertions (Phase 2 closure).
     { name: 'A24', drive: driveA24 },
+    // Plan 02-04 Task 2 A25: REQ-archive-export-latency / SPEC §10 #6.
+    // Page-side measures SAVE→ack via performance.now() bookends; host-side
+    // adds the dispatch→file-on-disk latency check via downloadsDir
+    // polling + mtime delta. Hard ceiling: 5000ms end-to-end. A25 owns
+    // its setupFreshRecording (clean latency measurement; not compounded
+    // with A24's still-pending state). The 11s segment-settle is NOT
+    // counted toward the 5s budget — only the SAVE dispatch.
+    { name: 'A25', drive: driveA25Wrapped },
   ];
 
   const buffers = { swConsole: handles.swConsole, offConsole: handles.offConsole };