fix: Resolve --exec double script execution bug

Scripts passed to --exec were executing twice because GameEngine
constructor ran scripts, and main.cpp created two GameEngine instances.

- Move exec_scripts from constructor to new executeStartupScripts() method
- Call executeStartupScripts() once after final engine setup in main.cpp
- Remove double-execution workarounds from tests
- Delete duplicate test_viewport_visual.py (flaky due to race condition)
- Fix test constructor syntax and callback signatures

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/GameEngine.h

@@ -146,6 +146,7 @@ public:
     void run();
     void sUserInput();
     void cleanup(); // Clean up Python references before destruction
+    void executeStartupScripts(); // Execute --exec scripts (called once after final engine setup)
     int getFrame() { return currentFrame; }
     float getFrameTime() { return frameTime; }
     sf::View getView() { return visible; }