v0.4.0: Theme engine — semantic colors, presets, dark/light variants

- Theme class with role→hex hash table, mode (dark/light)
- theme-color reader/writer (gethash based)
- define-preset macro with dark and light variants
- load-preset function with keyword lookup
- 2 built-in presets: default (gold) and nord
- 30+ semantic roles per preset (primary, accent, error, syntax-*, etc.)
- 9 theme tests: create, set/get, unknown, dark/light presets,
  nord, unknown-warn, switch-mode
- 57 total component tests, 100% GREEN

demo.lisp

@@ -0,0 +1,28 @@
+;; demo.lisp — minimal cl-tui demo
+(load "/root/quicklisp/setup.lisp")
+(ql:quickload :fiveam :silent t)
+(load "backend/package.lisp")
+(load "backend/classes.lisp")
+(load "backend/simple.lisp")
+(load "backend/modern.lisp")
+(load "layout/layout.lisp")
+(load "src/components/package.lisp")
+(load "src/components/dirty.lisp")
+(load "src/components/box.lisp")
+(load "src/components/text.lisp")
+(load "src/components/render.lisp")
+(in-package :cl-tui.box)
+
+;; Demo 1: Simple backend (ASCII)
+(let* ((b (make-simple-backend))
+       (bx (make-box :border-style :rounded :title " Hello World " :width 30 :height 5)))
+  (compute-layout (box-layout-node bx) 30 5)
+  (render bx b))
+
+;; Demo 2: Box with text inside
+(let* ((b (make-simple-backend))
+       (tx (make-text "This is cl-tui in action!" :width 28 :height 1)))
+  (setf (layout-node-direction (text-layout-node tx)) :column)
+  (compute-layout (text-layout-node tx) 28 1)
+  (render tx b)
+  (format t "~%~%"))