feat(lang): layers - module flow in layer app uses base, items

The modules of one layer use each other freely and may go round;
anything outside the layer is held to the module's uses, and a layered
module with no uses reaches nothing outside it. The cycle check walks
the graph with each layer as one node, so a cycle leaving a layer is
refused. ludic deps shows the layers and counts the largest cycle
without their own edges. Reseed.

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

examples/rejected/layer_reach/hud/index.ludic

@@ -0,0 +1,5 @@
+# hud/index.ludic - in layer app with no uses line: items is outside the layer, so it is refused
+module hud in layer app
+export function hud_lines() -> int {
+  return menu_title() + item_count()
+}