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>

selfhost/frontend/parse.ludic

@@ -1017,6 +1017,7 @@ function parse_one_decl() -> void {
   if is_id("registry") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].text == "of") { parse_registry(); return }   # L8
   if is_id("def") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].kind == TK_ID) { parse_def(); return }            # L8
   if is_id("view") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].text == "{") { parse_view(); return }            # L11
+  if is_id("component") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].text == "{") { parse_ui_component(); return }  # L11
   if is_id("var")    { push(prog, parse_var()); return }
   if is_id("const")  { push(prog, parse_const()); return }
   var is_unsafe = false
@@ -1514,4 +1515,5 @@ function parse_program() -> void {
   }
   registries_finish()                              # L8: each registry gets its defs
   views_finish()                                   # L11: each view gets its model and call
+  components_finish()                              # L11: each component gets its class
 }