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>

changes/defaults-views-templates.md

@@ -1,6 +1,6 @@
 bump: minor
 type: feature
-**Default parameters, views and templates.** A parameter can have a default (`pad: float = 8.0`).
+**Default parameters, components, views and templates.** A parameter can have a default (`pad: float = 8.0`).
 A call leaves out what it does not change, and may pass its first arguments by position and the
 rest by name.
 
@@ -8,6 +8,11 @@ A `view` declaration is the one bridge between a program and its UI. It names th
 template may read, the functions it may ask and the `on` events it may send, and it writes
 `view_<name>() -> UiView`.
 
+A `component Name { prop, state, fields, functions, on events }` declaration beside `Name.xml` and
+`Name.lss` is a UI component. Its template and styles are compiled in (with `@import` inlined), each
+mounted instance keeps its own props and state, styles are scoped to it, and a parent's `class`,
+`style` and `id` land on its root.
+
 `ludic.ui` is now a template runtime. Screens and components are XML files loaded at run time,
 with:
 - `{expression}` bindings;