Rebuild UI as a scriptable element tree, archive the old system

The old UI system (X-macro static element list, hand-authored C
screens/widgets/focus stack) is archived under archive/ rather than
deleted, since it's a useful reference during the rewrite.

New native UI element pool (src/dusk/ui): a flat UI_ELEMENTS[128] pool
of tagged-union elements (Label, Rectangle, Scripted), a real
persistent parent/child tree (children[8] per element, cycle- and
capacity-checked uiElementSetParent), always-fresh worldX/worldY
(cheap enough to recompute on every read, no dirty-flag cache needed),
and cascading dispose. Rendering stays manual/immediate: a scripted
element with no render() override auto-renders its children by
default, but overriding render() takes full control (an override must
call renderChildren() itself to opt back in) -- this is deliberately
preserved end to end via a render()-shadow trampoline so overriding
render() always keeps working the same way regardless of how a node
is reached.

New scripting layer (src/dusk/script/module/ui): UIElement/Label/
Rectangle JS classes (Label/Rectangle share UIElement's prototype via
manual chaining, not JS `extends`), exposing x/y/worldX/worldY/parent/
add()/remove()/render()/renderChildren()/dispose(). UI.add()/
UI.remove() manage top-level render roots, mutually exclusive with
being someone's child.

Also: Scene gains a lateUpdate() hook (called once per frame after
every other update, for things like camera-follow that need to react
to where everything else ended up); several duskrpg call sites
(cutscene items, entityinteractable, entityplayer) that depended on
the now-archived RPG textbox are stubbed to console output instead of
a dialogue box, pending the new UI reaching that far.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/scripts/OverworldScene.js

@@ -4,22 +4,26 @@ var TestPlane = require('./TestPlane.js');
 
 class OverworldScene {
   constructor() {
-    this.time = 0;
   }
 
   init() {
     this.player = new Player();
     this.plane = new TestPlane();
     this.camera = new PlayerCamera(this.player);
+
+    this.rect = new Rectangle();
+    this.rect.color = { r: 255, g: 0, b: 0, a: 255 };
+    this.rect.width = 100;
+    this.rect.height = 100;
+    UI.add(this.rect);
+    
+    this.label = new Label();
+    this.label.text = 'Hello World!';
+    this.rect.add(this.label);
   }
 
-  update() {
+  lateUpdate() {
     this.camera.update();
-
-    this.time += Time.delta;
-    if(this.time > 3.0) {
-      Scene.set(new OverworldScene());
-    }
   }
 
   dispose() {
@@ -27,9 +31,16 @@ class OverworldScene {
     this.player.dispose();
     this.plane.dispose();
 
+    // rect.dispose() cascades to label (added as its child) and removes
+    // itself from UI's render roots -- no separate UI.remove()/dispose()
+    // needed for either.
+    this.rect.dispose();
+
     this.camera = null;
     this.player = null;
     this.plane = null;
+    this.label = null;
+    this.rect = null;
   }
 }