feat(gui): Add SUB-WT widget toolkit, icon set, modal dialogs, six apps and a reworked window manager

Turns the desktop from a fixed set of hand-drawn panels into a real
environment applications can be written against.

SUB-WT immediate-mode toolkit (gui/gui_widgets.c):
  Buttons, labels, checkboxes, radios, sliders, list boxes, text fields, tab
  bars, scrollbars, progress bars, sparklines and badges. An app feeds the WM
  event into the toolkit and then calls widgets from its paint routine, so
  each call both draws the control and reports what the user did with it --
  no retained widget tree to keep in sync with window state.

Icons and dialogs:
  gui_icons.c holds 14 16x16 palette-indexed glyphs stored as hex text, so the
  art stays hand-editable and picks up theme changes for free; they scale by
  integer factors and can be drawn as tint stencils. gui_dialog.c adds modal
  info/warning/error/confirm dialogs that dim the desktop, take exclusive
  input, word-wrap on spaces and report results through a callback.

Applications (gui/gui_apps_ext.c):
  Settings (tabbed appearance/input/power), Task Manager (live process table
  over task_find_by_pid with memory and heap gauges), Kernel Log viewer (reads
  the dmesg ring, strips ANSI, colours by severity, scrolls), KTest Runner
  (drives the in-kernel suites and reports 21/21 with timing), Clock & Calendar
  (Zeller's congruence month grid) and a Text Editor.

Window manager (gui/gui_wm.c):
  Minimize/maximize/restore buttons, a corner resize grip, edge snapping, tile
  and cascade layouts, F5-F9 shortcuts, title truncation, and index-based
  enumeration so the taskbar no longer assumes window ids stay dense.

Desktop shell (gui/gui_desktop.c):
  Launcher icon grid, richer start menu, right-click context menu, taskbar
  buttons that minimize and restore, and a live CPU trace in the tray.

Two interaction bugs fixed along the way:
  - keyboard_get_key() blocks until a key arrives, so calling it every frame
    froze the compositor; it is now gated on keyboard_has_key(), and stale
    console input is drained before the first frame.
  - When the desktop consumed a click, the WM's press-edge detector kept the
    pre-press state and read the still-held button as a fresh press on the
    next frame, stealing focus from whatever the click had just opened.
    gui_wm_sync_button_state() keeps the two layers in step.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01L5yKJT4Wo2cV3rfPrwY5MP

README.md

@@ -41,6 +41,14 @@
 - 🧪 **In-Kernel Test Harness (`kernel/ktest.c`)**:
   - KUnit-style suites with assertion macros, per-case pass/fail reporting and timing.
   - **6 built-in suites, 21 cases, 735 assertions** covering rbtree, kfifo, hashtable, RCU, futex, wait queues, page cache and libcore — runnable from the shell (`ktest`) or the desktop KTest Runner.
+- 🖥️ **SUB-OS Graphical Desktop Environment (`gui/`)**:
+  - **SUB-WM Compositor**: 800x600 TrueColor double-buffered window manager with drag, corner resize, minimize/maximize/restore, edge snapping (left/right/maximize), tile and cascade layouts, and z-ordered painter's-algorithm compositing.
+  - **SUB-WT Widget Toolkit (`gui/gui_widgets.c`)**: Immediate-mode controls -- buttons, labels, checkboxes, radios, sliders, list boxes, text fields, tab bars, scrollbars, progress bars, sparklines and badges -- so an app rebuilds its interface from state each frame instead of maintaining a retained widget tree.
+  - **Icon Set (`gui/gui_icons.c`)**: 14 hand-drawn 16x16 palette-indexed glyphs, integer-scalable and tintable, driving the desktop grid, start menu and window chrome.
+  - **Modal Dialog Layer (`gui/gui_dialog.c`)**: Info, warning, error and confirm dialogs that dim the desktop and take exclusive input, with word-wrapped messages and callback results.
+  - **Desktop Shell (`gui/gui_desktop.c`)**: Gradient wallpaper with grid overlay, launcher icon grid, start menu, right-click context menu (tile / cascade / toggle grid), and a taskbar with per-window buttons, live CPU trace, memory badge and RTC clock.
+  - **Applications**: Terminal, File Explorer, System Monitor, Task Manager, Kernel Log viewer (live dmesg with severity colouring and scrollback), KTest Runner (drives the in-kernel suites and reports pass/fail), Text Editor, Calculator, Paint Studio, Clock & Calendar, Settings and About.
+  - **Keyboard Shortcuts**: `F5` cycle focus, `F6` cascade, `F7` tile, `F8` maximize, `F9` minimize, `Esc` exit to the kernel TTY.
 - ⚡ **Freestanding C++17 OOP Kernel Engine (`kernel/cpp/`)**:
   - **Bare-Metal C++ Runtime**: `operator new`/`delete`, sized deallocation, placement `new`, pure virtual handlers (`__cxa_pure_virtual`), and `.init_array` global constructor dispatcher.
   - **Modern Generic Containers**: Pure freestanding `Vector<T>`, `UniquePtr<T>` (with polymorphic converting constructors and custom deleters), dynamic `String`, and move semantics (`kernel::move`, `kernel::forward`).
@@ -210,9 +218,15 @@ make ARCH=armv8i
 make run ARCH=armv8i
 ```
 
-#### Graphical Window Emulation (Optional SDL/GTK GUI)
+#### Graphical Desktop Environment
+SUB-OS boots straight into the desktop. Pass `nogui`, `text`, `emergency` or
+`single` on the kernel command line to go directly to the kernel TTY instead;
+from inside the desktop, `Esc` (or Start -> Exit to Kernel TTY) drops to the
+same shell.
+
 ```bash
-make run-gui
+make run-gui     # Local SDL/GTK window
+make run-vnc     # Headless: VNC server on localhost:5900
 ```
 
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