fix: resolve import hang and index rebuild data loss bugs

Critical Bug Fixes (v3.43.2):

Bug #1 - Import Infinite Loop:
- Fix placeholder entity infinite loop in ImportCoordinator
- Use exact matching instead of fuzzy .includes() for entity names
- Search entities array (not rows) for existing placeholders
- Add duplicate relationship prevention in brain.relate()

Bug #2 - Index Rebuild File Discovery:
- Fix fileSystemStorage to scan sharded subdirectories
- Update getAllNodes() to use getAllShardedFiles()
- Update getAllEdges() to use getAllShardedFiles()
- Update getNodesByNounType() to use getAllShardedFiles()
- Fix getStorageStatus() to use O(1) persisted counts

Additional Improvements:
- Add brain.flush() API for explicit index persistence
- Make GraphAdjacencyIndex.flush() public
- Add auto-flush at end of import pipeline
- Update duplicate relationship test to expect deduplication

Files Modified:
- src/storage/adapters/fileSystemStorage.ts
- src/import/ImportCoordinator.ts
- src/brainy.ts
- src/graph/graphAdjacencyIndex.ts
- tests/unit/brainy/relate.test.ts

src/graph/graphAdjacencyIndex.ts

@@ -391,8 +391,9 @@ export class GraphAdjacencyIndex {
 
   /**
    * Flush dirty entries to cache
+   * CRITICAL FIX (v3.43.2): Now public so it can be called from brain.flush()
    */
-  private async flush(): Promise<void> {
+  async flush(): Promise<void> {
     if (this.dirtySourceIds.size === 0 && this.dirtyTargetIds.size === 0) {
       return
     }