feat(lang): UI components as files - component Name { ... } beside Name.xml and Name.lss

The compiler turns a component declaration into:
- a record of its props and state;
- a constructor, a props setter, a model and a call;
- a class, registered with ludic.ui at start.

Its template and styles are read from beside it and compiled in, with @import inlined, and a
missing template fails the build.

In the runtime:
- each mounted instance keeps its own props and state, and `set` in a template writes the state;
- styles are scoped to the component;
- class, style and id on a component's tag land on its root, styled by the parent's sheet too;
- ui_reload re-reads a component's files from disk and keeps instances.

The package's own module is now ludic_ui, so a program may call a directory of its own ui.

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

packages/ludic.ui/link.ludic

@@ -5,8 +5,16 @@ function lib_link(f: UiFile, t: UiTpl) -> void {
   t.sheet = f.sheet
   if not lk_element(t.tag) {
     t.comp = lk_find(f, t.tag, false)
+    if t.comp == null { t.cls = uc_find(t.tag) }
+    if t.cls != null {
+      lk_kids(f, t)
+      return
+    }
     if t.comp == null and lk_find(f, t.tag, true) != null { ui_err(`{t.where}: <{t.tag}>: that component is not exported - export="true" on it lets other files use it`) } else if t.comp == null { ui_err(`{t.where}: <{t.tag}>: there is no such element, and no component of that name here or exported by an import`) }
   }
+  lk_kids(f, t)
+}
+function lk_kids(f: UiFile, t: UiTpl) -> void {
   for i in 0 .. len(t.kids) { lib_link(f, t.kids[i]) }
 }
 function lk_element(tag: string) -> bool { return lk_in(LK_OWN, tag) or lk_in(EL_BOXES, tag) or lk_in(EL_TEXTS, tag) or nt_find(tag) != null }