feat(ui): ludic.ui is a UI framework - input, controls, render3d backend, the CSS a game needs

A game no longer writes a host:
- The runtime reads Input itself: focus and keyboard navigation (Tab, arrows, Enter/Space,
  autofocus), the pointer (hover, :active, click on release, drag), and scroll boxes with the
  wheel, a draggable scrollbar, clipping and scroll-into-view.
- HTML's controls are built in (button, checkbox, radio, range, select, text, key capture), made
  of plain parts a stylesheet styles, each reporting with on-change and event.value.
- ludic.ui/render3d.ludic draws with render3d's overlay: textures, named atlases (icon:NAME),
  nine-slice border-image, rounded rects and rings, clipping, and a scale.
- Hooks for the program's language, sounds and clock (ui_translator, ui_sounds, ui_clock).
- ui_dev: hot reload, errors on screen, LUDIC_UI_DUMP.
- CSS:
  - colours as #rgb / #rrggbbaa / rgb() / rgba() / names;
  - border-radius and outline (following the radius), box-shadow, background-image and a tinted
    border-image;
  - group opacity, @keyframes / animation, transition;
  - :focus, :focus-visible and :focus-within.
- HTML mixed content, and boolean attributes.
- render3d gains tex_width / tex_height, and the XML reader keeps text runs in order.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.ui/native.ludic

@@ -22,7 +22,6 @@ export function ui_native(tag: string, measure: fn(UiNode, float) -> void, draw:
   push(nt_list, nv)
 }
 function nt_find(tag: string) -> UiNative {
-  nt_defaults()
   for i in 0 .. len(nt_list) {
     if nt_list[i].tag == tag { return nt_list[i] }
   }
@@ -71,7 +70,15 @@ export function ui_attr(n: UiNode, key: string) -> Val {
   return Value.null()
 }
 export function ui_attr_text(n: UiNode, key: string) -> string { return vl_text(ui_attr(n, key)) }
+# a boolean attribute: present with no value is true, as HTML's are (<button autofocus>)
 export function ui_attr_on(n: UiNode, key: string) -> bool {
-  let v = ui_attr(n, key)
+  if n.akeys == null { return false }
+  for i in 0 .. len(n.akeys) {
+    if n.akeys[i] == key { return el_flag(n.vals[i]) }
+  }
+  return false
+}
+function el_flag(v: Val) -> bool {
+  if Value.kind(v) == 4 and Value.as_str(v) == "" { return true }
   return vl_truthy(v) and vl_text(v) != "false"
 }