Code quality, safety, stability, and cleanups.

- Replace header include guards with `#pragma once` and perform minor optimizations.
- Replaced traditional include guards with `#pragma once` for simplicity and to reduce boilerplate in all headers.
- Improved CLI line number handling with clamping and error messaging.
- Enhanced `chdir` error handling for macOS GUI builds.
- Removed redundant logic for GUI builds.
- Adjusted font constructor and registry to handle `const` data pointers consistently.

TestRenderer.h

@@ -1,9 +1,7 @@
 /*
  * TestRenderer.h - minimal renderer for testing (no actual display)
  */
-#ifndef KTE_TEST_RENDERER_H
-#define KTE_TEST_RENDERER_H
-
+#pragma once
 #include <cstddef>
 
 #include "Renderer.h"
@@ -31,6 +29,4 @@ public:
 
 private:
 	std::size_t draw_count_ = 0;
-};
-
-#endif // KTE_TEST_RENDERER_H
+};