fix: update tests for v5.1.0 API changes (VFS auto-init + property access)

BREAKING API CHANGES IN v5.1.0:
1. VFS now a property (brain.vfs) not method (brain.vfs())
2. VFS auto-initializes during brain.init() - no separate vfs.init() needed
3. Root directory (/) created automatically during brain.init()
4. Stricter UUID validation (32 hex chars required)
5. VFS readFile() returns Buffer (use .toString() for string)

TEST FIXES (106 tests fixed, 86% → 96.7% pass rate):
- Fixed brain.vfs() → brain.vfs in 17 test files
- Updated VFS initialization tests for auto-init behavior
- Fixed type-filtering test to account for VFS root directory
- Fixed UUID validation tests to use valid UUID format
- Updated VFS readFile expectations (Buffer → string conversion)

FILES UPDATED (19 test files):
- tests/unit/brainy-core.unit.test.ts (UUID validation)
- tests/unit/type-filtering.unit.test.ts (VFS root count)
- tests/unit/workshop-vfs-diagnostic.test.ts (VFS API)
- tests/vfs/vfs-initialization.unit.test.ts (auto-init behavior)
- tests/vfs/vfs.unit.test.ts (VFS API)
- tests/vfs/vfs-bug-fixes.unit.test.ts (VFS API)
- tests/integration/* (VFS API changes, 7 files)
- tests/manual/* (VFS API changes, 5 files)

TEST RESULTS:
- Before: 906/1051 passing (86%), 120 failures
- After: 1016/1051 passing (96.7%), 14 failures
- Critical systems: VFS (✓ ALL PASS), COW (✓ ALL PASS), Batch Ops (✓ 25/26)

Remaining 14 failures are edge cases (UUID validation) - will fix in patch.

tests/unit/brainy-core.unit.test.ts

@@ -89,16 +89,17 @@ describe('Brainy 3.0 Core (Unit Tests)', () => {
     })
 
     it('should handle non-existent IDs according to API contract', async () => {
-      const fakeId = 'non-existent-id'
-      
+      // Use valid UUID format (stricter validation in v5.1.0)
+      const fakeId = '00000000-0000-0000-0000-000000000000'
+
       expect(await brain.get(fakeId)).toBeNull()
-      
+
       // update should handle non-existent ID gracefully
-      await expect(brain.update({ 
-        id: fakeId, 
-        data: { test: 'data' } 
+      await expect(brain.update({
+        id: fakeId,
+        data: { test: 'data' }
       })).rejects.toThrow()
-      
+
       // delete should not throw for non-existent ID
       await expect(brain.delete(fakeId)).resolves.not.toThrow()
     })