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/text.lisp

@@ -18,7 +18,7 @@
     :underline underline :reverse reverse :dim dim
     :fg fg :bg bg))
 
-(defclass text ()
+(defclass text (dirty-mixin)
   ((layout-node :initform (make-layout-node) :accessor text-layout-node
      :initarg :layout-node)
    (content :initform "" :initarg :content :accessor text-content)