ci: add GitHub Actions workflow for unit, build, and e2e

Two jobs that run on every PR and on pushes to dev/master:

- unit (fast): npm ci + vitest. No emsdk, no wasm.
- build-and-e2e: clones submodules, sets up emsdk via
  mymindstorm/setup-emsdk@v14 (cached), builds wasm
  (npm run build), builds JS bundles (npm run build-es6),
  installs Chromium, runs Playwright. Uploads the
  playwright-report/ directory on failure for triage.

concurrency.cancel-in-progress lets a new push to the same PR
abort the previous run automatically.

No auto-publish on tag yet; release is still a manual npm publish.

Refs #29
Refs #31

Author: claude

.github/workflows/ci.yml

@@ -0,0 +1,65 @@
+name: CI
+
+on:
+  push:
+    branches: [dev, master]
+  pull_request:
+    branches: [dev, master]
+
+# Cancel superseded runs on the same branch / PR.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  unit:
+    name: Unit tests (Vitest)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: npm
+      - run: npm ci
+      - run: npm test
+
+  build-and-e2e:
+    name: Build wasm + JS, run Playwright
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: npm
+
+      - uses: mymindstorm/setup-emsdk@v14
+        with:
+          version: 3.1.61
+          actions-cache-folder: emsdk-cache
+
+      - run: npm ci
+
+      - name: Build wasm
+        run: npm run build
+
+      - name: Build JS bundles
+        run: npm run build-es6
+
+      - name: Install Playwright browsers
+        run: npx playwright install --with-deps chromium
+
+      - name: Run Playwright e2e
+        run: npm run test:e2e
+
+      - name: Upload Playwright report on failure
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: playwright-report
+          path: playwright-report/
+          retention-days: 7