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.lisp

@@ -0,0 +1,67 @@
+(in-package :cl-tui.box)
+
+;; ── Component Protocol ────────────────────────────────────────
+
+(defgeneric component-layout-node (component)
+  (:documentation "Return the layout-node for COMPONENT.")
+  (:method ((bx box)) (box-layout-node bx))
+  (:method ((tx text)) (text-layout-node tx)))
+
+(defgeneric component-children (component)
+  (:documentation "Return the children of COMPONENT, or nil.")
+  (:method ((bx box)) nil)
+  (:method ((tx text)) nil))
+
+(defgeneric component-parent (component)
+  (:documentation "Return the parent of COMPONENT, or nil.")
+  (:method ((c t)) nil))
+
+;; ── Rendering Pipeline ────────────────────────────────────────
+
+(defgeneric render (component backend)
+  (:documentation "Render COMPONENT at its computed position using BACKEND.")
+  (:method ((c t) backend)
+    (declare (ignore backend))
+    (values)))
+
+(defmethod render ((bx box) backend)
+  (render-box bx backend))
+
+(defmethod render ((tx text) backend)
+  (render-text tx backend))
+
+(defun render-screen (root backend)
+  "Render the component tree ROOT using BACKEND.
+  Computes layout for dirty branches, calls render on each component,
+  and wraps output in synchronized updates."
+  (let ((w (available-width root))
+        (h (available-height root)))
+    (begin-sync backend)
+    (render-node root backend w h)
+    (end-sync backend)))
+
+(defun render-node (node backend w h)
+  "Render a component NODE and its children."
+  (compute-layout (component-layout-node node) w h)
+  (render node backend)
+  (dolist (child (component-children node))
+    (render-node child backend w h)))
+
+(defun available-width (component)
+  "Return the available width for COMPONENT (or 80 as default)."
+  (let ((ln (component-layout-node component)))
+    (if ln (layout-node-width ln) 80)))
+
+(defun available-height (component)
+  "Return the available height for COMPONENT (or 24 as default)."
+  (let ((ln (component-layout-node component)))
+    (if ln (layout-node-height ln) 24)))
+
+;; ── Dirty Propagation ─────────────────────────────────────────
+
+(defun propagate-dirty (component)
+  "Mark COMPONENT and all ancestors dirty."
+  (mark-dirty component)
+  (let ((parent (component-parent component)))
+    (when parent
+      (propagate-dirty parent))))