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

src/components/package.lisp

@@ -24,5 +24,8 @@
    #:render #:render-screen #:render-node
    #:component-layout-node #:component-children #:component-parent
    #:available-width #:available-height
-   #:propagate-dirty))
+   #:propagate-dirty
+   ;; Theme engine
+   #:theme #:make-theme #:theme-mode
+   #:theme-color #:load-preset #:define-preset))
 (in-package :cl-tui.box)