Windowed GUI: wm module, input routing, and the paint app

- Add the wm window-manager facility per ADR 0004: overlapping
  pixel-surface windows, damage-rectangle compositing, WM-drawn title
  bar, focus/minimize/close, and F1-F4 keybindings
- Add gop shadow compositing primitives and vt::repaint_region so the
  console stays layer 0 beneath windows
- Route mouse (absolute coords, press/release edges) and keyboard
  (focus-aware input pump) to the focused window
- Add the rspaint drawing-program crate (pen, 8 colors, clear) and shell
  draw/windows/restore commands
- Extend TaskMsg with AppMouse/Focus/Close and document the design in
  CONTEXT.md + ADR 0004

1 files changed, 52 insertions, 0 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 48cb3a4..9fccbe4 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -77,6 +77,58 @@ Two-phase model matching output:
 The framebuffer address stays valid after `ExitBootServices` — the GPU's
 linear frame buffer is system memory that we own once boot services are gone.
 
+## Window
+
+A rectangular display surface owned by exactly one task and composited
+onto the 800×600 canvas by the window manager.  The global console (vt)
+becomes the first window, owned by the shell; further windows are spawned
+per task (a terminal, a drawing canvas, …).  Windows may overlap, so they
+carry a z-order.  Every window is a pixel surface: a terminal window is a
+text grid rendered into its own pixels, a drawing canvas draws pixels
+directly.  The ruling constraint is "stupid simple".
+
+## Chrome
+
+Window-manager-owned UI drawn on the canvas but not a window: the
+background/desktop fill, per-window title bars, and (later) a
+launcher/menu.  Chrome is not focusable and does not take input as a
+window.
+
+## Title bar
+
+Per-window chrome strip at the top of a window showing the window's title
+and its minimize and close buttons.  Drawn by the window manager, not the
+app.
+
+## Minimized
+
+A window state in which the window is not rendered (hidden); the app keeps
+running.  The inverse of the normal visible state.
+
+## Close
+
+The action that destroys a window and ends its app.
+
+## App
+
+A task that owns exactly one window and receives input events through its
+message queue.  A GUI program is an app; there is no separate "process"
+concept — the app is its task plus its window.  The console is app 0
+(window 0).
+
+## Focus
+
+The single window (if any) that receives keyboard and mouse input.  Focus
+is set by clicking a window, which also raises it; input is delivered to
+the focused app's message queue.  With no focused window, a click only
+sets focus.
+
+## Keybinding
+
+A global keyboard shortcut handled by the window manager before input is
+routed to the focused window.  It triggers a WM action (spawn a terminal,
+focus/restore the next window, close, …) and is never delivered to an app.
+
 ## Keyboard layout
 
 Scancode set 1, US QWERTY layout.  Hardcoded scancode→ASCII table.