feat: add onion architecture preset to boundary rules (#229)

Add onion preset with layers: domain-model → domain-services →
application → infrastructure. Enforces inward-only dependency
direction like the other presets.

Author: carlos-alm

src/boundaries.js

@@ -58,6 +58,10 @@ export const PRESETS = {
     layers: ['entities', 'usecases', 'interfaces', 'frameworks'],
     description: 'Inward-only dependency direction',
   },
+  onion: {
+    layers: ['domain-model', 'domain-services', 'application', 'infrastructure'],
+    description: 'Inward-only dependency direction',
+  },
 };
 
 // ─── Module Resolution ───────────────────────────────────────────────

tests/unit/boundaries.test.js

@@ -184,10 +184,11 @@ describe('validateBoundaryConfig', () => {
 // ─── PRESETS ─────────────────────────────────────────────────────────
 
 describe('PRESETS', () => {
-  test('all three presets defined', () => {
+  test('all four presets defined', () => {
     expect(PRESETS.hexagonal).toBeDefined();
     expect(PRESETS.layered).toBeDefined();
     expect(PRESETS.clean).toBeDefined();
+    expect(PRESETS.onion).toBeDefined();
   });
 
   test('each preset has layers array', () => {