infra: add tsconfig, typescript devDep, typecheck script, CI step

Adopt JSDoc + // @ts-check + `tsc --noEmit` as a CI/dev-only type
check. No emit step, no runtime dependency: shipped .js files stay
byte-identical, typescript is a devDependency only.

- tsconfig.json: allowJs + checkJs:false so only files carrying an
  explicit `// @ts-check` directive are checked; strict mode; DOM libs.
  `include` matches static/**/*.d.ts so the upcoming ambient contract
  is loaded into the program. moduleDetection is left at the default
  `auto` on purpose -- app.js/highway.js carry no import/export and
  must stay global scripts so cross-file globals resolve.
- package.json: typescript devDep + `npm run typecheck`.
- tests.yml: setup-node + `npm ci` + typecheck step before pytest.
  This also gives the existing JS plugin-API test step an explicit
  Node toolchain.

No file carries `// @ts-check` yet, so typecheck passes trivially.

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

.github/workflows/ci.yml

@@ -48,6 +48,17 @@ jobs:
           python -m pip install --upgrade pip
           pip install -r requirements.txt -r requirements-test.txt
 
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+
+      - name: Install Node dependencies
+        run: npm ci
+
+      - name: Typecheck (tsc --noEmit)
+        run: npm run typecheck
+
       - name: Run pytest
         run: pytest