ci(SundialKit): download iOS and watchOS platforms before archive

The archive-demo job selects Xcode 26.5 but the macos-26 image does
not ship the iOS/watchOS runtimes preinstalled, causing xcodebuild to
fail with "iOS 26.x is not installed". Add explicit -downloadPlatform
steps for iOS and watchOS (both demo apps embed watchOS targets).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: leogdion

.github/workflows/sundial-demo.yml

@@ -113,6 +113,12 @@ jobs:
       - name: Select Xcode
         run: sudo xcode-select -s "/Applications/Xcode_26.5.app"
 
+      - name: Download iOS platform
+        run: xcodebuild -downloadPlatform iOS
+
+      - name: Download watchOS platform
+        run: xcodebuild -downloadPlatform watchOS
+
       - name: Setup SSH for AppCerts repository
         uses: webfactory/ssh-agent@v0.9.0
         with: