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/env.ludic

@@ -13,6 +13,7 @@ export property UiEnv {
   on_acts: []UiAct = null
   on_envs: []UiEnv = null
   ctx: UiEnv = null              # what a <provide> above gave, seen through components too
+  inst: UiInst = null            # the Ludic component whose template this is
 }
 var ui_view: UiView = null
 var ui_errs: []string = new []string
@@ -54,10 +55,15 @@ function env_get(e: UiEnv, name: string) -> Val {
   ui_err(`no {name} here: not a loop's, a prop, a state or in the view`)
   return Value.null()
 }
-# `set name = value`: the nearest state that holds the name
+# `set name = value`: the nearest component's state of that name, or <state> that holds it
 function env_set(e: UiEnv, name: string, v: Val) -> void {
   var at = e
   while at != null {
+    if at.inst != null {
+      let args = Value.list()
+      Value.add(args, v)
+      if Value.kind(at.inst.cls.call(at.inst.ptr, `set:{name}`, args)) != 99 { return }
+    }
     if at.state != null and Value.has(at.state, name) != 0 {
       Value.put(at.state, name, v)
       return