feat(gui): Implement complete Graphical Desktop Environment, SUB-WM, and apps

- Create separate feature branch 'feat/gui-desktop'
- Implement double-buffered 2D graphics engine with alpha blending, gradients, rounded rects, and font rasterization in gui/gui_gfx.c and include/gui/gui_gfx.h
- Implement alpha-blended mouse pointer overlay in gui/gui_cursor.c and include/gui/gui_cursor.h
- Implement SUB-WM Window Manager with draggable titlebars, close/minimize buttons, drop shadows, and event routing in gui/gui_wm.c and include/gui/gui_wm.h
- Implement Cyberpunk/Nord desktop environment with wallpaper, taskbar, start menu popup, and system tray in gui/gui_desktop.c and include/gui/gui_desktop.h
- Implement built-in GUI apps: System Performance Monitor, GUI Terminal Emulator, File Explorer, Calculator, Paint Studio, and About dialog in gui/gui_apps.c and include/gui/gui_apps.h
- Add 'startx', 'gui', and 'desktop' applets in LazyBox suite
- Verified multi-architecture compilation across x86_64, AArch64, and ARMv8i

gui/gui.c

@@ -0,0 +1,11 @@
+// SUB-OS Graphical User Interface Subsystem
+#include <gui/gui.h>
+#include <kernel/printk.h>
+
+void gui_init(void) {
+    printk(KERN_INFO "GUI: SUB-OS Graphical Desktop Environment initialized (Ready)\n");
+}
+
+int gui_start_desktop(void) {
+    return gui_desktop_run();
+}