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/select.ludic

@@ -11,7 +11,7 @@ export property UiSel {
   pargs: []string = null
   nots: []UiSel = null
   prev: UiSel = null
-  comb: int = 0                 # 0: prev is an ancestor, 1: prev is the parent
+  comb: int = 0                 # prev is: 0 an ancestor, 1 the parent, 2 the sibling before, 3 any sibling before
 }
 function sel_new() -> UiSel {
   let s = new UiSel
@@ -36,6 +36,14 @@ function sel_parse(src: string, err: []string) -> UiSel {
       comb = 1
       continue
     }
+    if rd_op(r, "+") {
+      comb = 2
+      continue
+    }
+    if rd_op(r, "~") {
+      comb = 3
+      continue
+    }
     let c = sel_compound(r)
     c.prev = cur
     c.comb = comb