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.ludic

@@ -0,0 +1,9 @@
+# L3: a layer frees the modules in it from naming each other, not from naming anything else
+import "layer_reach/items"
+import "layer_reach/menu"
+import "layer_reach/hud"
+program LayerReach {
+  entry {
+    print(menu_rows())
+  }
+}