fix: return minimal stats with counts when statistics don't exist

Previously, when no statistics file existed (e.g., first container restart
after adding entities), getStatisticsData() returned null, causing HNSW
rebuild to see entityCount=0 even though entities existed.

This fix ensures all storage adapters return minimal StatisticsData with
totalNodes/totalEdges populated from in-memory counts when statistics files
don't exist yet.

Changes:
- gcsStorage.ts: Return minimal stats instead of null on 404 errors
- memoryStorage.ts: Return minimal stats when statistics is null
- opfsStorage.ts: Return minimal stats on read errors and null checks
- s3CompatibleStorage.ts: Return minimal stats when no statistics found
- fileSystemStorage.ts: Use in-memory counts in mergeStatistics() null case

This completes the GCS persistence fix started in v3.37.3 and ensures
container restarts work correctly in all cloud environments.

Fixes: GCS container restart hang during HNSW index rebuild

src/storage/adapters/memoryStorage.ts

@@ -682,7 +682,19 @@ export class MemoryStorage extends BaseStorage {
    */
   protected async getStatisticsData(): Promise<StatisticsData | null> {
     if (!this.statistics) {
-      return null
+      // CRITICAL FIX (v3.37.4): Statistics don't exist yet (first init)
+      // Return minimal stats with counts instead of null
+      // This prevents HNSW from seeing entityCount=0 during index rebuild
+      return {
+        nounCount: {},
+        verbCount: {},
+        metadataCount: {},
+        hnswIndexSize: 0,
+        totalNodes: this.totalNounCount,
+        totalEdges: this.totalVerbCount,
+        totalMetadata: 0,
+        lastUpdated: new Date().toISOString()
+      }
     }
 
     // Return a deep copy to avoid reference issues