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

cl-tui.asd

@@ -20,7 +20,8 @@
     ((:file "package")
      (:file "dirty")
      (:file "box" :depends-on ("package"))
-     (:file "text" :depends-on ("package" "box")))))
+     (:file "text" :depends-on ("package" "box"))
+     (:file "render" :depends-on ("package" "box" "text")))))
   :in-order-to ((test-op (test-op :cl-tui-tests))))
 
 (asdf:defsystem :cl-tui-tests
@@ -36,6 +37,7 @@
    (:module "src/components"
     :components
     ((:file "box-tests")
-     (:file "dirty-tests"))))
+     (:file "dirty-tests")
+     (:file "render-tests"))))
   :perform (test-op (o c)
              (uiop:symbol-call :cl-tui-backend-test '#:run-tests)))