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/migration-lock.test.ts

@@ -18,7 +18,7 @@
  * the production feature-detection reads it.
  */
 
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
 import { Brainy, NounType, MigrationInProgressError } from '../../src/index.js'
 import { GraphAdjacencyIndex } from '../../src/graph/graphAdjacencyIndex.js'
 
@@ -39,6 +39,12 @@ describe('Migration LOCK (#18) — coordinated 7.x→8.0 auto-upgrade', () => {
     await brain.init()
   })
 
+  afterEach(async () => {
+    // The "close() is not gated" test already closes `brain` itself as its
+    // own assertion — closing an already-closed brain is a safe no-op here.
+    await brain.close().catch(() => {})
+  })
+
   it('does not gate operations when no provider is migrating (fast path)', async () => {
     const id = await brain.add({ data: 'hello', type: NounType.Concept })
     expect(id).toBeTruthy()
@@ -130,6 +136,9 @@ describe('Migration LOCK (#18) — coordinated 7.x→8.0 auto-upgrade', () => {
       expect(e).toBeInstanceOf(MigrationInProgressError)
       expect(e.retryable).toBe(true)
       expect(typeof e.elapsedMs).toBe('number')
+    } finally {
+      // close() is proven not-gated by the test below — safe even mid-migration.
+      await shortBrain.close()
     }
   })