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/node.ludic

@@ -56,8 +56,12 @@ export property UiNode {
   lines: []string = null        # a wrapped text's lines
   var_names: []string = null    # its custom properties (--name), which its descendants inherit
   var_vals: []string = null
-  fg: int = -1
-  bg: int = -2
+  fg: int = -1                  # a colour is 0xRRGGBB with its alpha beside it; -1 inherits
+  fg_a: float = 1.0
+  bg: int = -2                  # -2: no background
+  bg_a: float = 1.0
+  border_a: float = 1.0
+  rfg_a: float = 1.0
   alpha: float = 1.0
   enabled: bool = true
   act: UiAct = null             # on-press, which the pointer and the renderer's button fire
@@ -76,5 +80,28 @@ export property UiNode {
   rsize: float = 0.0
   rfg: int = 0
   hovered: bool = false
+  focusable: bool = false       # a control, or anything with a tabindex: the keyboard reaches it
+  radius: float = 0.0           # border-radius
+  outline_w: float = 0.0        # outline: drawn outside the box, where a focus ring goes
+  outline_c: int = -1
+  outline_a: float = 1.0
+  outline_off: float = 0.0
+  scroll_y: float = 0.0         # how far a scroll box is scrolled
+  bar_c: int = -1               # scrollbar-color: its thumb, and its track
+  bar_track: int = -2
+  opts: []UiNode = null         # a select's options
+  bg_img: string = ""           # background-image: url(...), stretched over the box
+  bi_src: string = ""           # border-image: a nine-slice drawn instead of the background, tinted by it
+  bi_slice: float = 0.0
+  bi_width: float = 0.0
+  sh_x: float = 0.0             # box-shadow: an offset copy of the box, drawn under it
+  sh_y: float = 0.0
+  sh_c: int = -2
+  sh_a: float = 1.0
+  anim: string = ""             # animation: its @keyframes, how long, looping, back and forth
+  anim_dur: float = 0.0
+  anim_loop: bool = false
+  anim_alt: bool = false
+  tr_dur: float = 0.0           # transition: how long a change of opacity takes
 }