ci: install codecov coverage uploads (Tier-2 quality)

Free unlimited for public repos. Surfaces coverage delta in PR
comments. No account / token needed for public repo uploads.

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

Author: claude

.github/workflows/coverage.yml

@@ -0,0 +1,26 @@
+name: coverage
+
+on:
+  pull_request:
+    branches: [master, main]
+  push:
+    branches: [master, main]
+
+permissions:
+  contents: read
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm test -- --coverage || true
+      - uses: codecov/codecov-action@v4
+        with:
+          fail_ci_if_error: false