add simulator mode to desktop designer for testing apps

- Add --simulator flag to launch home screen showing discovered apps
- Create app discovery system to scan test-apps/ directory
- Build simulator home screen with dark phone-like UI
- Add Lua API: simulator.launchApp, simulator.goHome, simulator.getApps
- ESC key returns to home when inside an app
- Apps displayed with icons in grid layout

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

designer/CMakeLists.txt

@@ -71,6 +71,7 @@ add_executable(mosis-designer
     src/hot_reload.cpp
     src/platform_singleton.cpp
     src/desktop_sandbox.cpp
+    src/app_discovery.cpp
     src/testing/action_recorder.cpp
     src/testing/action_player.cpp
     src/testing/ui_inspector.cpp
@@ -111,3 +112,10 @@ add_custom_command(TARGET mosis-designer POST_BUILD
         ${CMAKE_CURRENT_SOURCE_DIR}/../src/main/assets
         $<TARGET_FILE_DIR:mosis-designer>/assets
 )
+
+# Copy simulator assets
+add_custom_command(TARGET mosis-designer POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy_directory
+        ${CMAKE_CURRENT_SOURCE_DIR}/assets/simulator
+        $<TARGET_FILE_DIR:mosis-designer>/simulator
+)