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>

examples/library/ui_anim.ludic

@@ -0,0 +1,27 @@
+# ui_anim.ludic — CSS animation and transition on ludic.ui's frame clock (sixty frames a second
+# when the renderer gives no clock of its own): a fade-in from @keyframes, a pulse that loops back
+# and forth, and a transition easing an opacity that changed.
+import "ludic.ui"
+program UiAnim {
+  numbers float
+  var dim: bool = false
+  view Anim {
+    dim = dim
+  }
+  const PAGE: string = "<ui><style>@keyframes fade { from { opacity: 0 } to { opacity: 1 } }\n@keyframes pulse { from { opacity: 1 } to { opacity: 0.5 } }\n.in { animation: fade 1s }\n.beat { animation: pulse 0.5s infinite alternate }\n.soft { transition: opacity 1s }\n.dim { opacity: 0.2 }</style><screen name='a'><p class='in'>a</p><p class='beat'>b</p><p class=\"soft {dim ? 'dim' : ''}\">c</p></screen></ui>"
+  function at() -> string {
+    ui_show("a", view_anim(), 0.0, 0.0, 200.0, 200.0)
+    let r: UiNode = ui_nodes("a", view_anim())
+    return `{int(r.children[0].alpha * 100.0)} {int(r.children[1].alpha * 100.0)} {int(r.children[2].alpha * 100.0)}`
+  }
+  entry {
+    ui_load_text(PAGE, "a.xml")
+    var out = at()
+    for i in 0 .. 29 { at() }
+    out = out + " | " + at()
+    dim = true
+    for i in 0 .. 29 { at() }
+    out = out + " | " + at()
+    print(out)
+  }
+}