ci: run native unit tests before firmware builds

Adds a "Unit tests" job running pio test -e native on ubuntu-latest.
The firmware build matrix waits for it, so a failing test also stops
release uploads.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/build.yaml

@@ -8,7 +8,28 @@ on:
     types: [published]
 
 jobs:
+  test:
+    name: Unit tests
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install PlatformIO
+        run: pip install platformio
+
+      - name: Run native unit tests
+        run: pio test -e native
+
   build:
+    # Don't build or publish firmware that fails its tests.
+    needs: test
     permissions: write-all
     name: ${{ matrix.name }}
     runs-on: ubuntu-latest