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/modules/layered/hud/index.ludic

@@ -0,0 +1,5 @@
+# hud/index.ludic - in layer app: it reaches menu back - a cycle, allowed inside the layer
+module hud in layer app
+export function hud_lines() -> int {
+  return menu_title() + 1
+}

examples/modules/layered/items/index.ludic

@@ -0,0 +1,5 @@
+# items/index.ludic - below the layer: it uses nothing
+module items uses
+export function item_count() -> int {
+  return 3
+}

examples/modules/layered/menu/index.ludic

@@ -0,0 +1,8 @@
+# menu/index.ludic - in layer app: it reaches hud (same layer) freely, and items through its uses
+module menu in layer app uses items
+export function menu_rows() -> int {
+  return item_count() + hud_lines()
+}
+export function menu_title() -> int {
+  return 1
+}

examples/modules/layers.ludic

@@ -0,0 +1,13 @@
+# layers.ludic — L3: `module menu in layer app uses items`. The modules of one layer use each other
+# freely and may go round (menu and hud use each other); anything outside the layer is held to the
+# module's `uses` - a layered module with no uses line may reach nothing outside its layer.
+#
+# Running it prints:  5
+import "layered/items"
+import "layered/menu"
+import "layered/hud"
+program Layers {
+  entry {
+    print(menu_rows())
+  }
+}