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_class.ludic

@@ -0,0 +1,45 @@
+# build_class.ludic - a Ludic component's instance built: the parent's attributes are its props (id,
+# class and style go on its root element instead, styled by the parent's sheet as well as its own),
+# its model is its props, state and fields, and its template reads nothing else
+function bd_class(k: UiTpl, e: UiEnv, key: string, out: []UiNode) -> void {
+  let cls = k.cls
+  uc_load(cls)
+  let it = in_get(key, cls)
+  let props = Value.object()
+  for i in 0 .. len(k.keys) {
+    let a = k.keys[i]
+    if a != "class" and a != "style" and a != "id" { Value.put(props, a, ui_eval(k.vals[i], e)) }
+  }
+  cls.props(it.ptr, props)
+  let ce = env_new(null)
+  ce.model = cls.model(it.ptr)
+  ce.inst = it
+  ce.ctx = e.ctx
+  ce.slot = k
+  ce.slot_env = e
+  ce.on_names = k.on_keys
+  ce.on_acts = k.on_acts
+  ce.on_envs = new []UiEnv
+  for i in 0 .. len(k.on_keys) { push(ce.on_envs, e) }
+  lc_note(key, k.on_keys, k.on_acts, e)
+  let at = len(out)
+  let root = new []UiTpl
+  push(root, cls.root)
+  bd_kids(root, ce, key, out)
+  if len(out) > at { bd_pass_down(out[at], k, e) }
+}
+# <Card class="wide" style="margin: 4px" id="c"> puts those on the card's own root element
+function bd_pass_down(n: UiNode, k: UiTpl, e: UiEnv) -> void {
+  let c = tpl_attr(k, "class")
+  if c != null {
+    let parts = lss_split(ui_eval_text(c, e), 32)
+    for i in 0 .. len(parts) {
+      if len(parts[i]) > 0 { push(n.classes, parts[i]) }
+    }
+  }
+  let id = tpl_attr(k, "id")
+  if id != null { n.id = ui_eval_text(id, e) }
+  if k.style != null { n.style2 = k.style }
+  n.sheet2 = k.sheet
+  n.env2 = e
+}