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_cycle.ludic

@@ -0,0 +1,9 @@
+# L3: a cycle is allowed inside a layer and nowhere else: items -> layer app -> items is refused
+import "layer_cycle/items"
+import "layer_cycle/menu"
+import "layer_cycle/hud"
+program LayerCycle {
+  entry {
+    print(menu_rows())
+  }
+}

examples/rejected/layer_cycle/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/rejected/layer_cycle/items/index.ludic

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+# items/index.ludic - uses hud, which is in layer app
+module items uses hud
+export function item_count() -> int {
+  return 3
+}

examples/rejected/layer_cycle/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/rejected/layer_reach.ludic

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+# 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())
+  }
+}

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()
+}

examples/rejected/layer_reach/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/rejected/layer_reach/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
+}