devops: test across different node.js versions

.github/workflows/tests.yml

@@ -15,6 +15,15 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
+        node: [24]
+        # Other supported Node versions run on Linux only.
+        include:
+          - os: ubuntu-latest
+            node: 20
+          - os: ubuntu-latest
+            node: 22
+          - os: ubuntu-latest
+            node: 26
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v6
@@ -24,10 +33,13 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:
-          node-version: 24
+          node-version: ${{ matrix.node }}
 
+      # pnpm 11 needs Node 22.13+, so use the standalone build with its own Node.
       - name: Setup PNPM
         uses: pnpm/action-setup@0e279bb959325dab635dd2c09392533439d90093 # v6.0.8
+        with:
+          standalone: true
 
       - name: Install dependencies
         run: pnpm i --frozen-lockfile