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

README.md

@@ -214,7 +214,7 @@ make run-gui
 | **Entertainment & Games** | `snake`, `matrix`, `calc`, `vtart` |
 | **Kernel & Tracing** | `lsmod`, `insmod`, `rmmod`, `slabinfo`, `trace`, `unshare`, `io_uring_test` |
 | **Diagnostics & Metrics**| `neofetch`, `uname`, `free`, `uptime`, `top`, `htop`, `ps`, `dmesg`, `vmstat`, `iostat` |
-| **System Control** | `clear`, `help`, `sleep`, `reboot`, `shutdown`, `poweroff`, `tty` |
+| **System Control & GUI** | `clear`, `help`, `sleep`, `reboot`, `shutdown`, `poweroff`, `tty`, `startx`, `gui`, `desktop` |
 
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