test(web): add opt-in reasoning chunk stress lane

vitest.web-stress.config.ts

@@ -0,0 +1,15 @@
+import tsconfigPaths from 'vite-tsconfig-paths'
+import { defineConfig } from 'vitest/config'
+import { vitestExecArgv } from './vitest.shared.ts'
+
+/** Opt-in browser performance lane; no default Vitest config includes *.stress.ts. */
+export default defineConfig({
+  plugins: [tsconfigPaths({ projects: ['./tsconfig.base.json'] })],
+  test: {
+    execArgv: vitestExecArgv,
+    include: ['apps/web/stress-tests/**/*.stress.ts'],
+    testTimeout: 600_000,
+    hookTimeout: 120_000,
+    fileParallelism: false,
+  },
+})