Phones: sideways only; back from the background the Vulkan surface is made again (it was lost), the work saved on the way out and carried on with on the next start if the phone closed it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

engine/engine.h

@@ -36,6 +36,9 @@ public:
     // A file dropped on the window, or opened with the program from the desktop (a Mac's Finder
     // hands it over this way, not on the command line).
     virtual void onFileDropped(const std::string& path) { (void)path; }
+    // Going into the background (a phone: another app, the home screen), or the system is short
+    // of memory: the work should be kept now, as the app may be closed while it's away.
+    virtual void onBackground() {}
     // Hot reload: called twice a second (hotreload 1) after the engine reloaded what changed
     // (`changed`: textures, models, sounds); the game reloads its own (scripts, the map).
     virtual void hotReload(const std::vector<std::string>& changed) { (void)changed; }