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

userland/lazybox/lazybox.c

@@ -41,6 +41,7 @@
 #include <drivers/virtio_gpu.h>
 #include <drivers/e1000e.h>
 #include <drivers/virtio_input.h>
+#include <gui/gui.h>
 #include <arch/arch.h>
 #include <kernel/printk.h>
 #include <lib/string.h>
@@ -1024,6 +1025,11 @@ static int applet_e1000e(int argc, char** argv) {
     return 0;
 }
 
+static int applet_startx(int argc, char** argv) {
+    (void)argc; (void)argv;
+    return gui_start_desktop();
+}
+
 static int applet_readelf(int argc, char** argv) {
     if (argc >= 2) {
         int fd = vfs_open(argv[1], 0);
@@ -2177,6 +2183,9 @@ static const lazybox_applet_t applets[] = {
     {"cpufreq",       applet_cpufreq,       "cpufreq [-g governor]",     "CPU frequency & P-State gov", "System"},
     {"gpuinfo",       applet_gpuinfo,       "gpuinfo",                   "VirtIO-GPU hardware status", "Virtualization"},
     {"e1000e",        applet_e1000e,        "e1000e",                    "Intel e1000e PCIe NIC stats", "Network"},
+    {"startx",        applet_startx,        "startx",                    "Start SUB-OS Desktop GUI",   "Desktop"},
+    {"gui",           applet_startx,        "gui",                       "Start SUB-OS Desktop GUI",   "Desktop"},
+    {"desktop",       applet_startx,        "desktop",                   "Start SUB-OS Desktop GUI",   "Desktop"},
 
     {NULL, NULL, NULL, NULL, NULL}
 };