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/package.lisp

@@ -19,5 +19,10 @@
    ;; Utilities (for tests)
    #:word-wrap #:split-string
    ;; Dirty tracking
-   #:dirty-mixin #:dirty-p #:mark-clean #:mark-dirty))
+   #:dirty-mixin #:dirty-p #:mark-clean #:mark-dirty
+   ;; Rendering pipeline
+   #:render #:render-screen #:render-node
+   #:component-layout-node #:component-children #:component-parent
+   #:available-width #:available-height
+   #:propagate-dirty))
 (in-package :cl-tui.box)