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>

examples/library/ui_reload.ludic

@@ -0,0 +1,27 @@
+# ui_reload.ludic — a template read from disk, changed there, and read again by ui_reload: the new
+# text shows at once, and the screen's state (a counter pressed before the change) is kept.
+import "ludic.ui"
+program UiReload {
+  numbers float
+  view Note {
+    title = "note"
+  }
+  const PATH: string = "build/ui_reload_example.xml"
+  const V1: string = "<ui><screen name='note'><state n='{0}'/><button on-click='set n = n + 1'>{title} {n}</button></screen></ui>"
+  const V2: string = "<ui><screen name='note'><state n='{0}'/><h1>{title} again {n}</h1></screen></ui>"
+  function shown() -> string {
+    let root: UiNode = ui_nodes("note", view_note())
+    return root.children[0].text
+  }
+  entry {
+    Fs.write_text(PATH, V1)
+    ui_load(PATH)
+    let root: UiNode = ui_nodes("note", view_note())
+    ui_press(root.children[0])
+    let before = shown()
+    let same = ui_reload()
+    Fs.write_text(PATH, V2)
+    let changed = ui_reload()
+    print(`{before} | reloaded {same} then {changed} | {shown()}`)
+  }
+}