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/transition.ludic

@@ -0,0 +1,31 @@
+# transition.ludic - `transition: opacity 0.2s`: when an element's opacity changes from one frame to
+# the next it eases to the new value over that time instead of jumping (opacity is the property a
+# game fades most; the others arrive as they are)
+var tr_keys: []string = new []string
+var tr_from: []float = new []float
+var tr_to: []float = new []float
+var tr_at: []float = new []float
+function tr_apply(n: UiNode) -> void {
+  if n.tr_dur <= 0.0 { return }
+  let now = an_now()
+  let want = n.alpha
+  for i in 0 .. len(tr_keys) {
+    if tr_keys[i] == n.key {
+      if tr_to[i] != want {
+        tr_from[i] = tr_value(i, now, n.tr_dur)
+        tr_to[i] = want
+        tr_at[i] = now
+      }
+      n.alpha = tr_value(i, now, n.tr_dur)
+      return
+    }
+  }
+  push(tr_keys, n.key)
+  push(tr_from, want)
+  push(tr_to, want)
+  push(tr_at, now)
+}
+function tr_value(i: int, now: float, dur: float) -> float {
+  let t = Math.clamp((now - tr_at[i]) / dur, 0.0, 1.0)
+  return tr_from[i] + (tr_to[i] - tr_from[i]) * t
+}