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/relate-duplicate-optimization.test.ts

@@ -18,7 +18,7 @@ describe('Duplicate Check Optimization', () => {
   })
 
   afterEach(async () => {
-    // Cleanup is automatic with memory storage
+    await brain.close()
   })
 
   it('should detect duplicate relationships using GraphAdjacencyIndex', async () => {