wip(0.S3): ui blocks, scene on enter/exit, hooks, machines over a state's field, namespace and engine-system injection; a reference out of a read-only state or a module let is read-only; deps counts writes into another module's state; the rejected examples hold states

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/frontend/component.ludic

@@ -60,6 +60,7 @@ function parse_ui_component() -> void {
   let head = node(N_VAR)
   head.s = px + "reg"
   head.ty = "int"
+  head.pos = -1                                  # the compiler's own, not a program's var
   push(g_cm_head, head)
   push(prog, head)
 }