CI: also build the app for the iOS Simulator

Catches changes that break the Xcode project (missing file references,
compile errors in the app target), which swift test on VisuStructCore
cannot see.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cp4EQgFQyGbM8MVLiWbJKQ

Author: claude

.github/workflows/core-tests.yml

@@ -1,4 +1,4 @@
-name: VisuStructCore tests
+name: CI
 
 on:
   push:
@@ -17,3 +17,17 @@ jobs:
         run: swift --version
       - name: Build and test
         run: swift test
+
+  app-build:
+    runs-on: macos-15
+    steps:
+      - uses: actions/checkout@v4
+      - name: Xcode version
+        run: xcodebuild -version
+      - name: Build app for iOS Simulator
+        run: |
+          xcodebuild build \
+            -project VisuStruct-iOS/VisuStructXcodeOpen/VisuStruct.xcodeproj \
+            -scheme VisuStruct \
+            -destination 'generic/platform=iOS Simulator' \
+            CODE_SIGNING_ALLOWED=NO