feat: enable within-file E2E test concurrency for 3x faster runs

Switch all E2E tests from serial test() to testConcurrentIfSelected()
so tests within each file run in parallel. Wall clock drops from ~18min
to ~6min (limited by the longest single test, not sequential sum).

The concurrent helper was already built in e2e-helpers.ts but never
wired up. Each test runs in its own describe block with its own
beforeAll/tmpdir — no shared state conflicts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/skill-e2e-qa-bugs.test.ts

@@ -4,7 +4,7 @@ import { outcomeJudge } from './helpers/llm-judge';
 import { judgePassed } from './helpers/eval-store';
 import {
   ROOT, browseBin, runId, evalsEnabled, selectedTests, hasApiKey,
-  describeIfSelected, describeE2E,
+  describeIfSelected, describeE2E, testConcurrentIfSelected,
   copyDirSync, setupBrowseShims, logCost, recordE2E, dumpOutcomeDiagnostic,
   createEvalCollector, finalizeEvalCollector,
 } from './helpers/e2e-helpers';
@@ -172,17 +172,17 @@ CRITICAL RULES:
   }
 
   // B6: Static dashboard — broken link, disabled submit, overflow, missing alt, console error
-  test('/qa finds >= 2 of 5 planted bugs (static)', async () => {
+  testConcurrentIfSelected('qa-b6-static', async () => {
     await runPlantedBugEval('qa-eval.html', 'qa-eval-ground-truth.json', 'b6-static');
   }, 360_000);
 
   // B7: SPA — broken route, stale state, async race, missing aria, console warning
-  test('/qa finds >= 2 of 5 planted SPA bugs', async () => {
+  testConcurrentIfSelected('qa-b7-spa', async () => {
     await runPlantedBugEval('qa-eval-spa.html', 'qa-eval-spa-ground-truth.json', 'b7-spa');
   }, 360_000);
 
   // B8: Checkout — email regex, NaN total, CC overflow, missing required, stripe error
-  test('/qa finds >= 2 of 5 planted checkout bugs', async () => {
+  testConcurrentIfSelected('qa-b8-checkout', async () => {
     await runPlantedBugEval('qa-eval-checkout.html', 'qa-eval-checkout-ground-truth.json', 'b8-checkout');
   }, 360_000);