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/main.cpp

@@ -213,6 +213,7 @@ int run_python_interpreter(const McRogueFaceConfig& config, int argc, char* argv
         delete engine;
         engine = new GameEngine(mutable_config);
         McRFPy_API::game = engine;
+        engine->executeStartupScripts();  // Execute --exec scripts ONCE here
         engine->run();
         McRFPy_API::api_shutdown();
         delete engine;