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>

packages/ludic.ui/build_more.ludic

@@ -0,0 +1,41 @@
+# build_more.ludic - <let name="{value}"/> names a value for the siblings after it, and
+# <provide name="{value}"> gives it to everything inside, components included (React's context)
+function bd_let(k: UiTpl, e: UiEnv) -> UiEnv {
+  let le = env_new(e)
+  for i in 0 .. len(k.keys) { env_bind(le, k.keys[i], ui_eval(k.vals[i], e)) }
+  return le
+}
+function bd_provide(k: UiTpl, e: UiEnv, key: string, out: []UiNode) -> void {
+  let cx = env_new(e.ctx)
+  for i in 0 .. len(k.keys) { env_bind(cx, k.keys[i], ui_eval(k.vals[i], e)) }
+  let pe = env_new(e)
+  pe.ctx = cx
+  bd_kids(k.kids, pe, key, out)
+}
+# a fragment's (or a template's) content: its elements, or its words as a text of their own
+function bd_group(k: UiTpl, e: UiEnv, key: string, out: []UiNode) -> void {
+  if k.text == null {
+    bd_kids(k.kids, e, key, out)
+    return
+  }
+  let t = new UiNode
+  t.kind = UI_TEXT
+  t.key = key
+  t.text = ui_eval_text(k.text, e)
+  t.env = e
+  t.children = new []UiNode
+  push(out, t)
+}
+# text inside text - <p>Hi <b>there</b></p>, <h3><slot/></h3> - reads as one line of it
+function el_inline(n: UiNode) -> void {
+  if n.kind != UI_TEXT or len(n.children) == 0 { return }
+  var s = n.text
+  for i in 0 .. len(n.children) {
+    let c = n.children[i]
+    el_inline(c)
+    if len(s) > 0 and len(c.text) > 0 { s = s + " " }
+    s = s + c.text
+  }
+  n.text = s
+  n.children = new []UiNode
+}