ci: guard main package publish until all platform packages are verified on npm

Prevents publishing gsd-pi to npm if any @gsd-build/engine-* optional
dependency is missing at the expected version. Avoids the failure mode
seen in 2.10.5 where the main package shipped before platform packages
were available, causing 'Cannot find module' errors on fresh installs.

.github/workflows/build-native.yml

@@ -145,6 +145,27 @@ jobs:
       - name: Wait for npm registry propagation
         run: sleep 30
 
+      - name: Verify platform packages are published
+        run: |
+          VERSION=$(node -p "require('./package.json').version")
+          echo "Verifying platform packages at version ${VERSION}..."
+          FAILED=0
+          for platform in darwin-arm64 darwin-x64 linux-x64-gnu linux-arm64-gnu win32-x64-msvc; do
+            PKG="@gsd-build/engine-${platform}"
+            PUBLISHED=$(npm view "${PKG}@${VERSION}" version 2>/dev/null || echo "")
+            if [ "${PUBLISHED}" = "${VERSION}" ]; then
+              echo "  ✓ ${PKG}@${VERSION}"
+            else
+              echo "::error::${PKG}@${VERSION} not found on npm (got: '${PUBLISHED}')"
+              FAILED=1
+            fi
+          done
+          if [ "${FAILED}" = "1" ]; then
+            echo "::error::One or more platform packages are missing from npm. Aborting main package publish to prevent broken installs."
+            exit 1
+          fi
+          echo "All platform packages verified."
+
       - name: Publish main package
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}