Update GitHub Actions workflow for v3 build

Split workflow into two jobs:
- build-test-v3: Builds and tests v3 directory (Node 18, npm)
- build-test-legacy: Builds legacy v0.1.x (only on master branch)

Changes:
- Updated to actions/checkout@v4 and actions/setup-node@v4
- v3 job runs on all branches/PRs
- Legacy job only runs on master branch
- v3 uses npm ci and working-directory for isolation
- Node 18 for v3 (matches package.json engines requirement)

This allows v3 development in branches while keeping legacy builds working on master.

Author: claude

.github/workflows/build-test.yml

@@ -9,15 +9,42 @@ on:
       - master
 
 jobs:
-  build-test:
+  build-test-v3:
+    name: Build and Test v3
     runs-on: ubuntu-latest
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Use Node.js
-        uses: actions/setup-node@v1
+        uses: actions/setup-node@v4
+        with:
+          node-version: 18.x
+
+      - name: Install v3 dependencies
+        working-directory: ./v3
+        run: npm ci
+
+      - name: Build v3
+        working-directory: ./v3
+        run: npm run build
+
+      - name: Run v3 tests
+        working-directory: ./v3
+        run: npm test
+
+  build-test-legacy:
+    name: Build and Test Legacy (v0.1.x)
+    runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/master'
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Use Node.js
+        uses: actions/setup-node@v4
         with:
           node-version: 14.x