v0.3.0: Rendering pipeline — render dispatch, tree walk, dirty propagation

- render generic function dispatches per component type
- render-screen entry point with sync wrapper
- render-node walks tree, computes layout, calls render
- component-layout-node generic (box/text methods)
- component-children/component-parent generics
- propagate-dirty marks component + ancestors dirty
- box and text now inherit from dirty-mixin
- 6 new tests: render dispatch, layout-node accessor, children,
  dirty propagation, available-width defaults
- 42 component tests, 100% GREEN

src/components/box.lisp

@@ -1,6 +1,6 @@
 (in-package :cl-tui.box)
 
-(defclass box ()
+(defclass box (dirty-mixin)
   ((layout-node :initform (make-layout-node) :accessor box-layout-node
      :initarg :layout-node)
    (border-style :initform :single :initarg :border-style