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/metadata-cold-read-guard.test.ts

@@ -15,7 +15,7 @@
  * The 8.0 JS index cold-loads correctly, so we simulate the cold native failure
  * mode by intercepting the provider's getIdsForFilter/rebuild.
  */
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
 import { Brainy, NounType, MetadataIndexNotReadyError } from '../../src/index.js'
 
 const V = () => Array.from({ length: 384 }, (_, i) => Math.sin(i * 0.1) + 0.001)
@@ -31,6 +31,10 @@ describe('Metadata cold-read guard (#venue silent-[])', () => {
     await brain.flush()
   })
 
+  afterEach(async () => {
+    await brain.close()
+  })
+
   it('warm brain: filtered find is correct and the guard does not rebuild', async () => {
     const mi = brain.metadataIndex
     let rebuilds = 0