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>

archive/test/ui/test_uitab.c

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#include "dusktest.h"
+#include "ui/widget/uitab.h"
+#include "util/memory.h"
+
+// uiTabInit requires a real font (FONT_DEFAULT) to build the label's
+// glyph cache, which needs a live GL context this test binary doesn't
+// have. uiTabRebuildBackground only reads the label's already-measured
+// width/height, so it's safe to set those directly and exercise it here.
+static void test_uiTabRebuildBackground_matchesLabelSize(void **state) {
+  uitab_t tab;
+  memoryZero(&tab, sizeof(uitab_t));
+  tab.label.width = 42;
+  tab.label.height = 12;
+
+  uiTabRebuildBackground(&tab);
+
+  assert_float_equal(tab.cachedBackground.min[0], 0.0f, 0.0001f);
+  assert_float_equal(tab.cachedBackground.min[1], 0.0f, 0.0001f);
+  assert_float_equal(tab.cachedBackground.max[0], 42.0f, 0.0001f);
+  assert_float_equal(tab.cachedBackground.max[1], 12.0f, 0.0001f);
+}
+
+int main(int argc, char **argv) {
+  const struct CMUnitTest tests[] = {
+    cmocka_unit_test(test_uiTabRebuildBackground_matchesLabelSize),
+  };
+
+  return cmocka_run_group_tests(tests, NULL, NULL);
+}