Extract GameEngine::doFrame() for Emscripten callback support

Refactors the main game loop to support both:
- Desktop: traditional blocking while(running) loop
- Browser: emscripten_set_main_loop_arg() callback (build-time conditional)

Changes:
- Add doFrame() method containing single-frame update logic
- Add isRunning() accessor for Emscripten callback
- run() now conditionally uses #ifdef __EMSCRIPTEN__ for loop selection
- Add emscriptenMainLoopCallback() static function

This is a prerequisite for Emscripten builds - browsers require
cooperative multitasking with callback-based frame updates.

Both normal and headless builds verified working.

Contributes to #158

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

src/GameEngine.h

@@ -235,6 +235,8 @@ public:
     sf::RenderTarget & getRenderTarget();
     sf::RenderTarget* getRenderTargetPtr() { return render_target; }
     void run();
+    void doFrame();  // Single frame update - extracted for Emscripten callback support
+    bool isRunning() const { return running; }  // Check if engine should continue running
     void sUserInput();
     void cleanup(); // Clean up Python references before destruction
     void executeStartupScripts(); // Execute --exec scripts (called once after final engine setup)