feat: Exit on first Python callback exception (closes #133)

By default, McRogueFace now exits with code 1 on the first unhandled
exception in timer, click, key, or animation callbacks. This prevents
repeated exception output that wastes resources in AI-driven development.

Changes:
- Add exit_on_exception config flag (default: true)
- Add --continue-after-exceptions CLI flag to preserve old behavior
- Update exception handlers in Timer, PyCallable, and Animation
- Signal game loop via McRFPy_API atomic flags
- Return proper exit code from main()

Before: Timer exceptions repeated 1000+ times until timeout
After: Single traceback, clean exit with code 1

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

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

src/GameEngine.cpp

@@ -289,6 +289,11 @@ void GameEngine::run()
         if (config.auto_exit_after_exec && timers.empty()) {
             running = false;
         }
+
+        // Check if a Python exception has signaled exit
+        if (McRFPy_API::shouldExit()) {
+            running = false;
+        }
     }
     
     // Clean up before exiting the run loop