test(architecture): ratchet top-level layer direction and mutual imports (#10584)

* test(architecture): ratchet top-level layer direction and mutual imports

Declare a layer for every top-level src/ directory and root source file
(domain, contracts, foundation, providers, application, composition,
adapters) in scripts/lib/module-layers.mjs. verify:module-boundaries now also
builds the production import graph between those units and compares it with
a counted baseline: 45 unit pairs import a higher layer (248 imports) and 23
unit pairs import each other. A new upward pair, more imports on a recorded
pair, a new mutual import, or an undeclared unit fails; shrinkage is
reported so the baseline can be lowered.

* fix(architecture): enforce layers for isolated units

scripts/lib/module-layers.d.mts

@@ -0,0 +1,21 @@
+export declare const LAYER_ORDER: readonly string[];
+export declare const UNIT_LAYERS: Readonly<Record<string, string>>;
+
+/** The top-level unit of a production `src/` file, or undefined. */
+export function sourceUnit(path: string): string | undefined;
+
+export interface LayerGraphFacts {
+  readonly undeclared: readonly string[];
+  readonly upward: Readonly<Record<string, number>>;
+  readonly cycles: readonly string[];
+}
+
+export function layerGraphFacts(
+  imports: readonly { readonly file: string; readonly target: string }[],
+  files?: readonly string[],
+): LayerGraphFacts;
+
+export function compareLayerBaseline(
+  facts: LayerGraphFacts,
+  baseline: Pick<LayerGraphFacts, "upward" | "cycles">,
+): { readonly failures: string[]; readonly lowerable: string[] };