test(hygiene): close every brain the unit suite creates

Each file opened one or more Brainy instances (beforeEach, or a small
per-test helper like migration-gate-family-scoped's module-level seed())
and never closed them. migration-gate-family-scoped.test.ts now tracks
every brain seed() hands back in a describe-scoped array drained by
afterEach, since the helper itself lives outside the describe block.

tests/unit/brainy-core.unit.test.ts

@@ -5,7 +5,7 @@
  * No mocks, no fakes, real implementation
  */
 
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
 import { Brainy } from '../../src/brainy.js'
 import { NounType } from '../../src/types/graphTypes.js'
 
@@ -21,6 +21,10 @@ describe('Brainy 3.0 Core (Unit Tests)', () => {
     await brain.init()
   })
 
+  afterEach(async () => {
+    await brain.close()
+  })
+
   describe('CRUD Operations', () => {
     it('should create items with add', async () => {
       const id = await brain.add({