feat(03-04): Task 1 — driveA32 host-side Page.metrics scaffolding + orchestrator wiring

A32 ships ~90 lines of best-effort RAM scaffolding per D-P3-04 +
RESEARCH Open Question 3 (recommended SHIP). Calls puppeteer.Page.metrics()
against the harness page and asserts JSHeapUsedSize is below the SPEC §10 #9
50 MB ceiling.

Page-realm scope is the load-bearing caveat (RESEARCH Pitfall 2): the MV3
service worker is a separate Puppeteer target with its own V8 isolate, so
Page.metrics() under-reports the operator-facing "extension background
RAM" measurement that §10 #9 actually requires. The binding §10 #9 gate
stays operator-driven (chrome://memory-internals OR chrome://extensions
service-worker memory display) and is recorded in Plan 03-05 VERIFICATION.md
human_verification block.

Mandatory diagnostic line emitted on EVERY run regardless of pass/fail:
"NOTE: page-realm only; SW context measurement requires
chrome://memory-internals operator verification per D-P3-04."
printAssertionResult prints diagnostics to stdout so the operator sees
the caveat in the live UAT trace, never confusing automation GREEN with
full §10 #9 closure (T-03-04-01 Repudiation mitigation).

Host-side only — no page-side assertA32, no setupFreshRecording, no
SAVE, no archive parse. driveA32 takes only `page` (no downloadsDir),
so the orchestrator pushes it bare in the drivers array without a wrapped
const. Tier-1 FORBIDDEN_HOOK_STRINGS inventory unchanged at 12 entries
(Page.metrics is host-side puppeteer; not bundled).

Empirical: UAT harness 32/32 → 33/33 GREEN; A32.1 PASS (JSHeapUsedSize=
1909924 bytes); A32.2 PASS (1.82 MB << 50 MB). Tier-1 unit-gate 13/13
sub-tests GREEN; 12 strings × 0 hits each in dist/. vitest 171/171 GREEN.

Closes:
- Plan 03-04 must_have 'puppeteer.Page.metrics() returns a JSHeapUsedSize
  value (>= 0) for the harness page realm' (A32.1)
- Plan 03-04 must_have 'JSHeapUsedSize for the harness page realm is
  below 50 MB' (A32.2)
- Plan 03-04 must_have 'Driver emits an explicit diagnostic line: NOTE:
  page-realm only' (Pitfall 2 gate — leads diagnostics array)
- Plan 03-04 must_have 'UAT harness exits 0 with 32 + 1 = 33/33
  assertions GREEN' (empirical 33/33)

tests/uat/lib/harness-page-driver.ts

@@ -2314,3 +2314,93 @@ export async function driveA31(
     error: pageResult.error,
   };
 }
+
+/* ─── Plan 03-04 — driveA32 (RAM scaffolding best-effort) ──────────── */
+
+/** RAM ceiling per SPEC §10 #9 + CON-ram-ceiling. */
+const A32_RAM_CEILING_BYTES = 50 * 1024 * 1024;
+/** Bytes-per-MB factor for diagnostic copy. */
+const A32_BYTES_PER_MB = 1024 * 1024;
+
+/**
+ * Drive A32 (Plan 03-04 — SPEC §10 #9 RAM best-effort per D-P3-04).
+ *
+ * Reads puppeteer.Page.metrics() against the harness page and asserts
+ * JSHeapUsedSize is below the 50 MB ceiling. This is informational
+ * scaffolding ONLY:
+ *
+ *   - RESEARCH Pitfall 2: Page.metrics is page-realm only. The MV3
+ *     service worker is a separate Puppeteer target with its own V8
+ *     isolate; page.metrics() does not aggregate across workers/iframes.
+ *   - The page-realm value reported here is NOT the operator-facing
+ *     "extension background RAM" measurement that SPEC §10 #9 requires.
+ *   - The binding §10 #9 gate lives in Plan 03-05 VERIFICATION.md
+ *     `human_verification` block (operator runs chrome://memory-internals
+ *     OR chrome://extensions service-worker memory display).
+ *
+ * Why ship this anyway (per RESEARCH Open Question 3):
+ *   - Low cost (~30 lines; single API call; no new bundle surface).
+ *   - Exercises the Page.metrics API end-to-end so Phase 4 (programmatic
+ *     RAM measurement upgrade) inherits a working scaffold.
+ *   - Provides a sanity floor — if the harness page-realm heap ever
+ *     blows past 50 MB, something has gone catastrophically wrong in
+ *     the test infrastructure itself (not necessarily a §10 #9 regression
+ *     in production).
+ *
+ * The diagnostic line about page-realm scope MUST be emitted regardless
+ * of pass/fail per Pitfall 2.
+ *
+ * @param page - The harness page from `launchHarnessBrowser`.
+ * @returns AssertionRecord with 2 checks (heap returned + heap < 50 MB)
+ *          + explicit page-realm-only diagnostic.
+ */
+export async function driveA32(page: Page): Promise<AssertionRecord> {
+  const checks: CheckRecord[] = [];
+  const diagnostics: string[] = [];
+
+  // Pitfall 2 gate: emit the page-realm caveat BEFORE any other diagnostic
+  // so it leads in the structured output (the operator sees it first).
+  diagnostics.push(
+    'NOTE: page-realm only; SW context measurement requires chrome://memory-internals operator verification per D-P3-04.',
+  );
+
+  let metricsErr: string | null = null;
+  let jsHeapBytes = -1;
+  let jsHeapTotal = -1;
+  try {
+    const metrics = await page.metrics();
+    jsHeapBytes = metrics.JSHeapUsedSize ?? -1;
+    jsHeapTotal = metrics.JSHeapTotalSize ?? -1;
+  } catch (err) {
+    metricsErr = err instanceof Error ? err.message : String(err);
+  }
+
+  const jsHeapMB = jsHeapBytes >= 0 ? jsHeapBytes / A32_BYTES_PER_MB : -1;
+  diagnostics.push(`A32 JSHeapUsedSize=${jsHeapBytes} bytes (${jsHeapMB.toFixed(2)} MB)`);
+  diagnostics.push(`A32 JSHeapTotalSize=${jsHeapTotal} bytes`);
+  if (metricsErr !== null) {
+    diagnostics.push(`A32 Page.metrics threw: ${metricsErr}`);
+  }
+
+  checks.push({
+    name: 'A32.1: Page.metrics returned a JSHeapUsedSize value >= 0',
+    expected: '>= 0',
+    actual: jsHeapBytes,
+    passed: jsHeapBytes >= 0,
+  });
+  checks.push({
+    name: `A32.2: Page-realm JS heap < ${A32_RAM_CEILING_BYTES / A32_BYTES_PER_MB} MB (NOTE: scaffolding only; SW context excluded per D-P3-04)`,
+    expected: `< ${A32_RAM_CEILING_BYTES / A32_BYTES_PER_MB} MB`,
+    actual: jsHeapMB >= 0 ? `${jsHeapMB.toFixed(2)} MB` : 'unavailable',
+    passed: jsHeapBytes >= 0 && jsHeapBytes < A32_RAM_CEILING_BYTES,
+  });
+
+  const passed = checks.every((c) => c.passed);
+  return {
+    passed,
+    name: 'A32 — RAM scaffolding (best-effort; page-realm only per D-P3-04 / SPEC §10 #9)',
+    checks,
+    diagnostics,
+    error: metricsErr ?? undefined,
+  };
+}