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/main.cpp

@@ -44,6 +44,11 @@ int run_game_engine(const McRogueFaceConfig& config)
     if (Py_IsInitialized()) {
         McRFPy_API::api_shutdown();
     }
+
+    // Return exception exit code if a Python exception signaled exit
+    if (McRFPy_API::shouldExit()) {
+        return McRFPy_API::exit_code.load();
+    }
     return 0;
 }
 
@@ -181,9 +186,13 @@ int run_python_interpreter(const McRogueFaceConfig& config, int argc, char* argv
         
         // Run the game engine after script execution
         engine->run();
-        
+
         McRFPy_API::api_shutdown();
         delete engine;
+        // Return exception exit code if signaled
+        if (McRFPy_API::shouldExit()) {
+            return McRFPy_API::exit_code.load();
+        }
         return result;
     }
     else if (config.interactive_mode) {
@@ -207,6 +216,10 @@ int run_python_interpreter(const McRogueFaceConfig& config, int argc, char* argv
         engine->run();
         McRFPy_API::api_shutdown();
         delete engine;
+        // Return exception exit code if signaled
+        if (McRFPy_API::shouldExit()) {
+            return McRFPy_API::exit_code.load();
+        }
         return 0;
     }