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>

changes/module-layers.md

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+bump: minor
+type: feature
+**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 still held to the module's `uses` (a
+layered module with no `uses` reaches nothing outside its layer), and a cycle is refused unless it
+stays inside one layer. `ludic deps` shows the layers and counts the largest cycle without the edges
+inside them, printing the count with them beside it.