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/attr_place.ludic

@@ -0,0 +1,23 @@
+# attr_place.ludic - position and its insets, z-index, overflow, white-space and text-overflow
+function el_place_prop(n: UiNode, key: string, v: Val) -> bool {
+  let s = vl_text(v)
+  if key == "position" {
+    n.pos = 0
+    if s == "relative" { n.pos = 1 }
+    if s == "absolute" { n.pos = 2 }
+    if s == "fixed" { n.pos = 3 }
+  } else if key == "top" { n.top = el_inset(v) } else if key == "right" { n.right = el_inset(v) } else if key == "bottom" { n.bottom = el_inset(v) } else if key == "left" { n.left = el_inset(v) } else if key == "inset" {
+    el_sides(v)
+    n.top = el_s0
+    n.right = el_s1
+    n.bottom = el_s2
+    n.left = el_s3
+  } else if key == "z-index" { n.z = int(el_num(s)) } else if key == "overflow" or key == "overflow-y" {
+    if (s == "auto" or s == "scroll" or s == "hidden") and n.kind == UI_BOX { n.kind = UI_SCROLL }
+  } else if key == "white-space" { n.nowrap = s == "nowrap" or s == "pre" } else if key == "text-overflow" { n.ellipsis = s == "ellipsis" } else { return false }
+  return true
+}
+function el_inset(v: Val) -> float {
+  if vl_text(v) == "auto" { return UI_AUTO }
+  return el_len(v)
+}