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/tpl.ludic

@@ -40,13 +40,15 @@ function tpl_read(x: Xml, file: string) -> UiTpl {
       push(t.vals, tpl_value(v, err))
     }
   }
-  for i in 0 .. Xml.child_count(x) { push(t.kids, tpl_read(Xml.child(x, i), file)) }
+  if tp_mixed(x) { tp_read_mixed(t, x, file, err) } else {
+    for i in 0 .. Xml.child_count(x) { push(t.kids, tpl_read(Xml.child(x, i), file)) }
+  }
   if t.tag == "style" {
     t.raw = Xml.text(x)
     return t
   }
   let words = tpl_words(Xml.text(x))
-  if len(words) > 0 and len(t.kids) == 0 { t.text = ex_text(words, err) }
+  if len(words) > 0 and Xml.child_count(x) == 0 { t.text = ex_text(words, err) }
   for i in 0 .. len(err) { ui_err(`{t.where}: <{t.tag}> {err[i]}`) }
   return t
 }