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>

examples/library/ui_component_parts/Counter.ludic

@@ -0,0 +1,9 @@
+# Counter.ludic — a component: what it takes (props), keeps (state), works out and does
+component Counter {
+  prop label: string
+  prop step: int = 1
+  state count: int = 0
+  doubled: int = count * 2
+  function big() -> bool { return count > 3 }
+  on bump() { count += step }
+}