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/neural/signals/EmbeddingSignal.test.ts

@@ -13,10 +13,11 @@ describe('EmbeddingSignal', () => {
     signal = new EmbeddingSignal(brain)
   })
 
-  afterEach(() => {
+  afterEach(async () => {
     signal.clearCache()
     signal.clearHistory()
     signal.resetStats()
+    await brain.close()
   })
 
   describe('initialization', () => {