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/render-tests.lisp

@@ -0,0 +1,47 @@
+(in-package :cl-tui-box-test)
+
+(defun make-capturing-backend ()
+  (let* ((s (make-string-output-stream))
+         (b (make-modern-backend :output-stream s)))
+    (values b s)))
+
+(test render-generic-dispatches-box
+  "render dispatches to render-box for box instances"
+  (multiple-value-bind (b s) (make-capturing-backend)
+    (let ((bx (make-box :border-style :single :width 10 :height 5)))
+      (compute-layout (box-layout-node bx) 10 5)
+      (render bx b)
+      (is (search "┌" (get-output-stream-string s)) "box renders border"))))
+
+(test render-generic-dispatches-text
+  "render dispatches to render-text for text instances"
+  (multiple-value-bind (b s) (make-capturing-backend)
+    (let ((tx (make-text "Hello" :width 10 :height 1)))
+      (compute-layout (text-layout-node tx) 10 1)
+      (render tx b)
+      (is (search "Hello" (get-output-stream-string s)) "text renders content"))))
+
+(test component-layout-node-works
+  "component-layout-node returns the right slot for each type"
+  (let ((bx (make-box)) (tx (make-text "")))
+    (is (typep (component-layout-node bx) 'layout-node))
+    (is (typep (component-layout-node tx) 'layout-node))))
+
+(test component-children-returns-nil
+  "Leaf components have no children"
+  (let ((bx (make-box)) (tx (make-text "")))
+    (is (null (component-children bx)))
+    (is (null (component-children tx)))))
+
+(test propagate-dirty-marks-component
+  "propagate-dirty marks the component dirty"
+  (let ((c (make-box)))
+    (mark-clean c)
+    (is-false (dirty-p c) "should be clean after mark-clean")
+    (propagate-dirty c)
+    (is-true (dirty-p c) "should be dirty after propagate-dirty")))
+
+(test available-width-defaults
+  "available-width returns 0 for components without explicit width"
+  (let ((c (make-box)))
+    (is (= (available-width c) 0))))