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/metadata-provider-contract.test.ts

@@ -18,7 +18,7 @@
  * exercised by cor's combined matrix); they inject probe/spy hooks onto the live JS
  * metadata index, which has neither method by default.
  */
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
 import { Brainy } from '../../../src/brainy'
 import { NounType } from '../../../src/types/graphTypes'
 
@@ -34,6 +34,10 @@ describe('metadata-provider contract wiring (getIdsForFilter opts)', () => {
     mi = (brain as any).metadataIndex
   })
 
+  afterEach(async () => {
+    await brain.close()
+  })
+
   it('RETIRED: a read never calls probeConsistency() / self-heals via detectAndRepairCorruption — that is the read-triggered dark rebuild the health-gate law forbids', async () => {
     let probes = 0
     let repairs = 0