test(hygiene): close every brain the vfs unit suite creates

Each file opened a Brainy per test (beforeEach) and never closed it.

tests/vfs/vfs-bulkwrite-race.unit.test.ts

@@ -12,7 +12,7 @@
  * other operations in parallel batches.
  */
 
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
 import { Brainy } from '../../src/brainy.js'
 import { VirtualFileSystem } from '../../src/vfs/VirtualFileSystem.js'
 
@@ -30,6 +30,10 @@ describe('VFS bulkWrite Race Condition Fix', () => {
     await vfs.init()
   })
 
+  afterEach(async () => {
+    await brain.close()
+  })
+
   describe('operation ordering', () => {
     it('should create directories before files when mixed in same batch', async () => {
       // This is the exact scenario that triggered the race condition: