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/utils/metadataIndex-array-bound.test.ts

@@ -48,6 +48,10 @@ describe('the indexable-array bound', () => {
     await brain.init()
   })
 
+  afterEach(async () => {
+    await brain.close()
+  })
+
   describe('BELOW the bound: the array indexes, every element of it', () => {
     it('the eleven-element array that used to vanish is searchable', async () => {
       // ELEVEN — one over the old silent limit, the whole shape of the defect.