ci(x): typecheck test files — the gap vitest and the build tsconfigs both miss

Test files were typechecked by nothing: tsconfig.build.json excludes
them (correctly — they must not land in dist), and vitest transforms
with esbuild, which strips types without checking them. That gap is
where the review's two tsc breaks and the fixture drift accumulated.

Adds 'typecheck' scripts per package (dev tsconfigs, which include
src/**/*.test.ts; renderer reuses its existing tsc -b), a root
orchestrator mirroring the test script, and a CI step in the vitest
job. All three packages are currently clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/x-tests.yml

@@ -34,6 +34,12 @@ jobs:
       - name: Run apps/x Vitest suites
         run: npm test
 
+      # Typechecks with the dev tsconfigs, which include test files —
+      # the build tsconfigs exclude them and vitest strips types without
+      # checking, so this is the only gate that sees type errors in tests.
+      - name: Typecheck apps/x packages
+        run: npm run typecheck
+
   apps-x-electron-package:
     name: apps/x Electron package smoke test
     runs-on: ubuntu-latest