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/controls_build.ludic

@@ -0,0 +1,86 @@
+# controls_build.ludic - a control's parts, made as plain child boxes a stylesheet can style:
+#   input[type=checkbox|radio]  .ui-label  .ui-track > .ui-knob
+#   input[type=range]           .ui-label  .ui-track > .ui-fill, .ui-thumb   .ui-value
+#   input[type=text|key]        .ui-label  .ui-field > .ui-value (+ .ui-caret while focused)
+#   select                      .ui-label  .ui-prev  .ui-value  .ui-next   (its <option>s hidden)
+function ct_part(n: UiNode, cls: string, text: string) -> UiNode {
+  let p = new UiNode
+  p.key = `{n.key}/{cls}`
+  p.tag = "span"
+  p.env = n.env
+  p.sheet = n.sheet
+  p.children = new []UiNode
+  p.classes = new []string
+  push(p.classes, cls)
+  p.akeys = new []string
+  p.avals = new []string
+  p.vals = new []Val
+  if text != "" {
+    p.kind = UI_TEXT
+    p.text = text
+  }
+  return p
+}
+function ct_build(n: UiNode) -> void {
+  n.dir = UI_ROW
+  n.focusable = true
+  let t = ui_attr_text(n, "type")
+  let label = ui_attr_text(n, "label")
+  if label != "" { push(n.children, ct_part(n, "ui-label", ui_tr(label))) }
+  if n.tag == "select" {
+    ct_build_select(n)
+    return
+  }
+  if t == "checkbox" or t == "radio" {
+    let track = ct_part(n, "ui-track", "")
+    push(track.children, ct_part(n, "ui-knob", ""))
+    push(n.children, track)
+  } else if t == "range" {
+    ct_build_range(n)
+  } else {
+    let field = ct_part(n, "ui-field", "")
+    push(field.children, ct_part(n, "ui-value", ct_field_text(n)))
+    if fc_key == n.key and t != "key" { push(field.children, ct_part(n, "ui-caret", "")) }
+    push(n.children, field)
+  }
+}
+function ct_build_range(n: UiNode) -> void {
+  let lo = Value.as_float(ui_attr(n, "min"))
+  var hi = Value.as_float(ui_attr(n, "max"))
+  if hi <= lo { hi = lo + 1.0 }
+  let v = Value.as_float(ui_attr(n, "value"))
+  let track = ct_part(n, "ui-track", "")
+  let fill = ct_part(n, "ui-fill", "")
+  fill.w_pct = Math.clamp((v - lo) / (hi - lo), 0.0, 1.0) * 100.0
+  push(track.children, fill)
+  push(track.children, ct_part(n, "ui-thumb", ""))
+  push(n.children, track)
+  push(n.children, ct_part(n, "ui-value", ui_attr_text(n, "value")))
+}
+function ct_build_select(n: UiNode) -> void {
+  let opts = n.children
+  n.children = new []UiNode
+  let label = ui_attr_text(n, "label")
+  if label != "" { push(n.children, ct_part(n, "ui-label", ui_tr(label))) }
+  var shown = ""
+  let at = ct_option_at(n, opts)
+  if at >= 0 { shown = opts[at].text }
+  push(n.children, ct_part(n, "ui-prev", "<"))
+  push(n.children, ct_part(n, "ui-value", shown))
+  push(n.children, ct_part(n, "ui-next", ">"))
+  n.opts = opts
+}
+# which option the select's value names: an option's value="...", else its place
+function ct_option_at(n: UiNode, opts: []UiNode) -> int {
+  let v = ui_attr_text(n, "value")
+  for i in 0 .. len(opts) {
+    let ov = ui_attr_text(opts[i], "value")
+    if ov == v or (ov == "" and string(i) == v) { return i }
+  }
+  return -1
+}
+function ct_field_text(n: UiNode) -> string {
+  if ui_attr_text(n, "type") != "key" { return ui_attr_text(n, "value") }
+  if ct_capture == n.key { return ui_tr("press a key...") }
+  return Input.key_label(Value.as_int(ui_attr(n, "value")))
+}