chore(01-11): wave-0 — install puppeteer + tsx, add vite.test.config + Tier-1 hook-leak grep gate

Task 1 of Plan 01-11 (Puppeteer UAT harness).

- npm install --save-dev puppeteer@^25.0.2 tsx@^4 @types/node
  resolved: puppeteer@25.x, tsx@4.22.1, @types/node@25.8.0
  pulls ~150MB Chromium binary at install time (T-1-11-03 — accepted,
  package-lock pins resolved hashes via @puppeteer/browsers).
- package.json scripts: add build:test + test:uat (per RESEARCH §10
  two-bundle orchestration); existing dev/build/preview/test untouched.
- vite.test.config.ts: extends ./vite.config.ts via mergeConfig with
  mode:'test' + build.outDir:'dist-test' + emptyOutDir:true. Verified
  npm run build:test produces dist-test/ in 7.93s; npm run build keeps
  producing dist/ in 7.67s (no clobber).
- tsconfig.json `include: ["src"]` already covers src/test-hooks/**/*
  via wildcard — no edit needed.
- tests/background/no-test-hooks-in-prod-bundle.test.ts: Tier-1 gate
  mirroring sw-bundle-import.test.ts's execFile pattern. Greps the
  BUILT dist/ tree for 5 forbidden hook surfaces (one `it` per surface
  for granular failure isolation): __mokoshTest, simulateUserStop,
  getSegmentCount, setCurrentStream, setSegmentCountGetter. All 5
  surfaces absent today (RED-then-GREEN polarity inverted — the gate
  is GREEN now and MUST stay GREEN after Task 2 lands the hooks).
  SKIP_BUILD=1 escape hatch for developer iteration.
- .gitignore: add dist-test/ (no point versioning generated test bundle).

Verification:
- npx tsc --noEmit: exit 0
- npm run build: exit 0; dist/ populated (375.37 kB SW chunk)
- npm run build:test: exit 0; dist-test/ populated (identical chunk sizes —
  the gated dynamic imports do not land until Task 2; this commit only
  proves the two-bundle plumbing)
- SKIP_BUILD=1 npx vitest run tests/background/no-test-hooks-in-prod-bundle.test.ts:
  6/6 GREEN (1 build-sanity + 5 forbidden-surface)
- SKIP_BUILD=1 npx vitest run (full suite): 89/89 GREEN
  (83 baseline + 6 new Tier-1 surfaces = 89)

Working-tree cleanup: a stale 5.4 MB tests/fixtures/last_30sec.webm
(unrelated operator smoke regen present at session spawn) was stashed
before running the baseline — it caused the webm-playback test to time
out at 5s. After stashing back to HEAD's 1.9 MB fixture, baseline passes
cleanly. Not committing the fixture restoration here (pre-existing
working-tree state, not part of Task 1).

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

vite.test.config.ts

@@ -0,0 +1,43 @@
+// vite.test.config.ts — Plan 01-11 two-bundle separation.
+//
+// Extends the production `./vite.config.ts` with two delta knobs:
+//   1. `mode: 'test'` — Vite statically replaces `import.meta.env.MODE`
+//      everywhere in the input source with the string literal `'test'`.
+//      The gated dynamic imports in src/background/index.ts +
+//      src/offscreen/recorder.ts (Plan 01-11 Task 2) take the form
+//      `if (import.meta.env.MODE === 'test') { await import('../test-hooks/...'); }`.
+//      With mode='test' the comparison resolves to a live branch and
+//      Rollup KEEPS the dynamic import; with the default mode='production'
+//      the comparison is a static dead branch and Rollup tree-shakes the
+//      `await import` away entirely (verified by the Tier-1 grep gate
+//      `tests/background/no-test-hooks-in-prod-bundle.test.ts`).
+//   2. `build.outDir: 'dist-test'` + `emptyOutDir: true` — emit to a
+//      SEPARATE directory so a `npm run build` immediately after this
+//      build does not clobber. Puppeteer harness consumes this path via
+//      `puppeteer.launch({ enableExtensions: [<abs-path-to-dist-test>] })`.
+//
+// References:
+//   - Vite mergeConfig: https://vite.dev/guide/api-javascript.html#mergeconfig
+//   - Vite environment variables: https://vite.dev/guide/env-and-mode.html
+//   - Rollup tree-shaking literal-comparison dead branches:
+//     https://rollupjs.org/plugin-development/#how-rollup-handles-dynamic-imports
+
+import { defineConfig, mergeConfig, type UserConfigExport } from 'vite';
+import baseConfig from './vite.config';
+
+const baseAsExport: UserConfigExport = baseConfig;
+
+export default defineConfig(({ command, mode }) =>
+  mergeConfig(
+    typeof baseAsExport === 'function'
+      ? baseAsExport({ command, mode, isPreview: false, isSsrBuild: false })
+      : baseAsExport,
+    {
+      mode: 'test',
+      build: {
+        outDir: 'dist-test',
+        emptyOutDir: true,
+      },
+    },
+  ),
+);