feat(01-13): wave-3A — add get-display-surface bridge op (A3 prereq) + extend Tier-1 grep gate

Scope: prerequisite step for Wave 3A's A3 assertion (displaySurface=monitor
verification). The page→offscreen bridge gains a new op so the harness can
query the active stream's `getSettings().displaySurface` without needing
direct offscreen.evaluate access (impossible by-construction; the only
cross-isolate path is chrome.runtime.sendMessage).

Bridge op contract (`src/test-hooks/offscreen-hooks.ts`):
  - Protocol: { type: '__mokoshOffscreenQuery', op: 'get-display-surface' }
  - Response: { displaySurface: string|null }
    • null when no current stream (recording not active)
    • 'monitor' when installFakeDisplayMedia's monkey-patched
      getSettings() reports it (production code in
      src/offscreen/recorder.ts enforces this same value — tears down
      stream + throws 'wrong-display-surface' otherwise).
  - Failure: { ok: false, error: <message> } only on getSettings throw.

Tier-1 grep gate extension (`tests/background/no-test-hooks-in-prod-bundle.test.ts`):
  - FORBIDDEN_HOOK_STRINGS: 8 → 9 entries.
  - Added: 'get-display-surface' (the literal bridge-op string;
    matches the production-bundle absence invariant — the offscreen-hooks
    module is tree-shaken in production builds by the Vite mode gate in
    src/offscreen/recorder.ts top-of-module).

Verification:
  - npx tsc: clean
  - npm run build: clean (dist/ 4 chunks; no offscreen-hooks artifact)
  - npm run build:test: clean (dist-test/ adds offscreen-hooks-DfWtG71P.js, 2.38kB)
  - SKIP_BUILD=1 vitest run no-test-hooks-in-prod-bundle.test.ts → 10/10 GREEN
    (1 build-sanity + 9 forbidden-string checks; production bundle hook-free)
  - SKIP_BUILD=1 vitest run (full) → 93/93 GREEN
    (Wave 0+1+2 baseline 92 + 1 from the 9th grep-gate string)
  - npx tsx tests/uat/a6.test.ts → A6 5/5 GREEN
    (lib-driven path preserved; bridge op addition does not interfere)

Wave 3A continuation: assertA1/A2/A3/A4 land in the next commit which
wires the harness-page surface + driver wrappers + harness.test.ts
orchestrator. This commit is the bridge prerequisite — keeping the
bridge-op extension atomic + the grep-gate extension atomic so the
'production bundle hook-free' invariant is provable BEFORE the page-side
surface lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/test-hooks/offscreen-hooks.ts

@@ -300,6 +300,10 @@ globalThis.__mokoshTest = {
 //   op='install-fake-display-media' → { ok: true } OR { ok: false, error }
 //   op='dispatch-ended' → { ok: true } OR { ok: false, error: 'no stream' }
 //   op='has-stream' → { hasStream: boolean }
+//   op='get-display-surface' → { displaySurface: string|null } OR { ok: false, error }
+//     — Plan 01-13 Wave 3A A3 contract. Returns the active track's
+//       `getSettings().displaySurface` value (monkey-patched to 'monitor'
+//       by `installFakeDisplayMedia`); returns null when no stream is live.
 // Unknown ops respond { ok: false, error: 'unknown-op' }.
 //
 // The bridge handler MUST run BEFORE the production offscreen bridge
@@ -349,6 +353,35 @@ chrome.runtime.onMessage.addListener((rawMessage, _sender, sendResponse) => {
     sendResponse({ hasStream: currentStream !== null });
     return false;
   }
+  if (op === 'get-display-surface') {
+    // Plan 01-13 Wave 3A A3 contract — return the active track's
+    // displaySurface so the harness can verify the offscreen-hooks
+    // monkey-patched getSettings() correctly reports 'monitor'.
+    // Production code in src/offscreen/recorder.ts enforces this same
+    // invariant (tears down + throws 'wrong-display-surface' otherwise),
+    // so if recording is live the value is guaranteed monitor — the
+    // harness asserts === 'monitor' for explicit empirical verification.
+    try {
+      if (currentStream === null) {
+        sendResponse({ displaySurface: null });
+        return false;
+      }
+      const track = currentStream.getVideoTracks()[0];
+      if (track === undefined) {
+        sendResponse({ displaySurface: null });
+        return false;
+      }
+      const settings = track.getSettings();
+      const displaySurface = settings.displaySurface ?? null;
+      sendResponse({ displaySurface });
+    } catch (err) {
+      sendResponse({
+        ok: false,
+        error: err instanceof Error ? err.message : String(err),
+      });
+    }
+    return false;
+  }
   sendResponse({ ok: false, error: 'unknown-op' });
   return false;
 });