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/build.ludic

@@ -14,7 +14,7 @@ function bd_kids(kids: []UiTpl, e0: UiEnv, key: string, out: []UiNode) -> void {
       e = bd_let(k, e)
       continue
     }
-    if k.tag == "fragment" or (k.tag == "template" and tpl_attr(k, "slot") == null) { bd_group(k, e, ck, out) } else if k.tag == "provide" { bd_provide(k, e, ck, out) } else if k.tag == "template" { continue } else if k.tag == "if" {
+    if k.tag == "fragment" or k.tag == "#text" or (k.tag == "template" and tpl_attr(k, "slot") == null) { bd_group(k, e, ck, out) } else if k.tag == "provide" { bd_provide(k, e, ck, out) } else if k.tag == "template" { continue } else if k.tag == "if" {
       last_if = bd_test(k, e)
       if last_if { bd_kids(k.kids, e, ck, out) }
     } else if k.tag == "else" {