ci(#227): add windows build job to build.yml

CI never compiled Windows, the primary test bench (RAK4631 / COM12 /
USB serial). Add a windows job mirroring the existing platform jobs:
runs-on windows-latest, flutter-action stable, pub get, flutter build
windows --release --no-pub.

Advisory to start (not in branch-protection required checks); flip to
required after 5 green runs (#226). The first run validates the
windows-latest env against the local bench (C++ ATL, Developer Mode,
llamadart/vulkan, flserial); fixes applied per the #226 plan.

Part of #226.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -76,3 +76,14 @@ jobs:
           cache: true
       - run: flutter pub get
       - run: flutter build web --release --no-pub
+
+  windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: subosito/flutter-action@v2
+        with:
+          channel: "stable"
+          cache: true
+      - run: flutter pub get
+      - run: flutter build windows --release --no-pub