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_controls.ludic

@@ -0,0 +1,71 @@
+# ui_controls.ludic — ludic.ui's own controls, driven as a player would drive them: Tab and the
+# arrows move the focus, Enter presses and flips, the arrows move a range and step a select, typing
+# fills a text field and Backspace takes a letter back, a key field takes the next key, the pointer
+# drags a range, and the wheel scrolls a box. Every change reaches the component as an event.
+import "ludic.ui"
+import "ui_controls_parts/Settings.ludic"
+program UiControls {
+  numbers float
+  function frame(i: UiInput) -> UiNode {
+    ui_input(i)
+    ui_show("Settings", null, 0.0, 0.0, 400.0, 400.0)
+    let root: UiNode = ui_nodes("Settings", null)
+    ui_place(root, 0.0, 0.0, 400.0, 400.0)
+    return root
+  }
+  function press(tab: bool, down: bool, right: bool, enter: bool) -> UiInput {
+    let i = new UiInput
+    i.tab = tab
+    i.down_key = down
+    i.right = right
+    i.enter = enter
+    return i
+  }
+  function state(root: UiNode) -> string {
+    let page = root.children[0]
+    let last = page.children[len(page.children) - 1]
+    return last.text
+  }
+  entry {
+    ui_backend(new UiBackend)
+    var root = frame(new UiInput)
+    root = frame(press(true, false, false, false))
+    root = frame(press(false, false, false, true))
+    root = frame(press(false, true, false, false))
+    root = frame(press(false, false, false, true))
+    root = frame(press(false, true, false, false))
+    root = frame(press(false, false, true, false))
+    root = frame(press(false, false, true, false))
+    root = frame(press(false, true, false, false))
+    root = frame(press(false, false, true, false))
+    root = frame(press(false, true, false, false))
+    let typed = new UiInput
+    typed.typed = "lovelace"
+    root = frame(typed)
+    let back = new UiInput
+    back.backspace = true
+    root = frame(back)
+    root = frame(press(false, true, false, false))
+    root = frame(press(false, false, false, true))
+    let k = new UiInput
+    k.key = 70
+    root = frame(k)
+    print(state(root))
+    let range = root.children[0].children[2]
+    let track = range.children[1]
+    let at = new UiInput
+    at.x = track.x + track.cw * 0.8
+    at.y = track.y + 2.0
+    at.down = true
+    root = frame(at)
+    root = frame(new UiInput)
+    let list = root.children[0].children[6]
+    let w = new UiInput
+    w.x = list.x + 5.0
+    w.y = list.y + 5.0
+    w.wheel = -1.0
+    root = frame(w)
+    root = frame(new UiInput)
+    print(`{state(root)}; scrolled {int(root.children[0].children[6].scroll_y)}`)
+  }
+}