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>

tests/demo/demo_main.py

@@ -14,9 +14,6 @@ from mcrfpy import automation
 import sys
 import os
 
-# Note: Engine runs --exec scripts twice - we use this to our advantage
-# First run sets up scenes, second run's timer fires after game loop starts
-
 # Add parent to path for imports
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))