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/input.ludic

@@ -0,0 +1,62 @@
+# input.ludic - what the player did this frame, read once from the runtime's Input: the pointer, its
+# button, the wheel, the keys a UI answers and what was typed. A test or a host can set it instead.
+export property UiInput {
+  x: float = -1.0
+  y: float = -1.0
+  down: bool = false
+  wheel: float = 0.0
+  tab: bool = false
+  shift: bool = false
+  up: bool = false
+  down_key: bool = false
+  left: bool = false
+  right: bool = false
+  enter: bool = false
+  space: bool = false
+  escape: bool = false
+  backspace: bool = false
+  typed: string = ""
+  key: int = 0                  # any key pressed this frame, for a key-capture field
+}
+var in_now: UiInput = new UiInput
+var in_was_down: bool = false
+var in_given: bool = false
+function in_read() -> void {
+  if in_given {
+    in_given = false
+    return
+  }
+  in_was_down = in_now.down
+  let i = new UiInput
+  if ui_be != null and ui_be.input {
+    i.x = float(Input.mouse_x())
+    i.y = float(Input.mouse_y())
+    i.down = Input.mouse_down(0)
+    i.wheel = float(Input.wheel())
+    i.tab = Input.key_pressed(Key.Tab)
+    i.shift = Input.key_down(Key.Shift)
+    i.up = Input.key_pressed(Key.Up)
+    i.down_key = Input.key_pressed(Key.Down)
+    i.left = Input.key_pressed(Key.Left)
+    i.right = Input.key_pressed(Key.Right)
+    i.enter = Input.key_pressed(Key.Enter)
+    i.space = Input.key_pressed(Key.Space)
+    i.escape = Input.key_pressed(Key.Escape)
+    i.backspace = Input.key_pressed(Key.Backspace)
+    i.typed = Input.text()
+    i.key = in_any_key()
+  }
+  in_now = i
+}
+function in_any_key() -> int {
+  for k in 8 .. 167 {
+    if Input.key_pressed(k) { return k }
+  }
+  return 0
+}
+# this frame's input, given by a test or a host that reads its own
+export function ui_input(i: UiInput) -> void {
+  in_was_down = in_now.down
+  in_now = i
+  in_given = true
+}