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/screen_class.ludic

@@ -0,0 +1,36 @@
+# screen_class.ludic - a Ludic component shown as a whole screen
+# a Ludic component shown as a screen: an instance at the top, keyed by its name
+function fr_class_nodes(cls: UiClass) -> UiNode {
+  lc_begin()
+  in_build += 1
+  ui_view = null
+  uc_load(cls)
+  let root = fr_root(cls.name, cls.sheet)
+  let use = new UiTpl
+  use.tag = cls.name
+  use.cls = cls
+  use.keys = new []string
+  use.vals = new []UiExpr
+  use.on_keys = new []string
+  use.on_acts = new []UiAct
+  use.kids = new []UiTpl
+  let e = env_new(null)
+  e.model = Value.object()
+  root.env = e
+  bd_class(use, e, cls.name, root.children)
+  in_sweep(cls.name)
+  cs_tree(root)
+  return root
+}
+function fr_root(key: string, sheet: UiSheet) -> UiNode {
+  let root = new UiNode
+  root.key = key
+  root.tag = "screen"
+  root.sheet = sheet
+  root.children = new []UiNode
+  root.classes = new []string
+  root.akeys = new []string
+  root.avals = new []string
+  root.vals = new []Val
+  return root
+}