feat(ui): ludic.ui grows toward HTML, CSS and React (Phase 0.U1-U4)

- Natives: ui_native(tag, measure, draw) makes an element the program draws itself; ui_fire
  runs its on-<event> with event.value. input, select and textarea have simple defaults.
- React:
  - keyed <each>, <let>, <provide> context through components, <fragment>;
  - named slots, default props on <component>;
  - on-mount / on-unmount;
  - inline text inside text elements.
- CSS:
  - custom properties and var();
  - position relative/absolute/fixed with insets and z-index;
  - em/rem/vw/vh and @media;
  - wrapping text and ellipsis, overflow;
  - + and ~ combinators, :nth-child(an+b), :checked, :active.
- Developing: errors with file:line, ui_errors(), ui_reload() keeping state, and an
  inspector-style ui_dump.
- view fields infer the type of a literal or a named function's result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/native/xml.ludic

@@ -21,6 +21,7 @@ property Xml {
   akeys: []pointer           # attribute names
   avals: []pointer           # attribute values (entity-decoded)
   kids:  []Xml               # child elements, in document order
+  at:    int = 0             # where its '<' is in the text, for an error that names a line
 }
 
 function xml_new(tag: pointer) -> Xml {
@@ -243,9 +244,11 @@ function xp_attrs(p: XP, node: Xml) -> void {
 
 # parse one element (cursor on its opening '<'). Recurses for children.
 function xp_element(p: XP) -> Xml {
+  let at = p.i
   p.i += 1                                           # skip '<'
   let name = xp_name(p)
   let node = xml_new(name)
+  node.at = at
   xp_attrs(p, node)
   # self-closing '/>'
   if p.i < p.n and p.s[p.i] == '/' {                       # '/'