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

Makefile

@@ -277,11 +277,13 @@ CONFIG_SRCS-y += drivers/char/tty.c drivers/char/keyboard.c drivers/char/serial.
                  drivers/input/mouse.c drivers/power/acpi.c drivers/power/cpufreq.c drivers/rtc/rtc.c drivers/sound/speaker.c \
                  drivers/net/e1000e.c drivers/virtio/virtio_gpu.c drivers/virtio/virtio_input.c
 
-# Userland
+# Userland & GUI Desktop Subsystem
 ifeq ($(CONFIG_USERLAND_LAZYBOX), y)
     CONFIG_SRCS-y += userland/lazybox/lazybox.c userland/lazybox/shell.c userland/lazybox/sh.c userland/lazybox/nano.c userland/lazybox/snake.c userland/lazybox/tree.c
 endif
 
+CONFIG_SRCS-y += gui/gui.c gui/gui_gfx.c gui/gui_cursor.c gui/gui_wm.c gui/gui_desktop.c gui/gui_apps.c
+
 # Deduplicate
 CONFIG_SRCS-y := $(sort $(CONFIG_SRCS-y))