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/build.ludic

@@ -71,6 +71,10 @@ function bd_slot(k: UiTpl, e: UiEnv, key: string, out: []UiNode) -> void {
   }
 }
 function bd_comp(k: UiTpl, e: UiEnv, key: string, out: []UiNode) -> bool {
+  if k.cls != null {
+    bd_class(k, e, key, out)
+    return true
+  }
   if k.comp == null { return false }
   let c = k.comp
   let ce = env_new(null)