perf: defer HNSW persistence during addMany() batch operations

addMany() now temporarily switches the HNSW index to deferred persist
mode during the batch loop. Previously, each add() triggered ~16-20
saveHNSWData calls for modified neighbors (each a read→gzip→atomic-write
cycle). For 450 items this was ~8,100 individual storage writes.

With deferred mode, dirty node IDs are collected in a Set during the
batch and flushed once at the end — deduplicating repeated neighbor
updates. A node modified by insert #3 and again by insert #47 is
written only once.

Also adds setPersistMode() to TypeAwareHNSWIndex, propagating mode
changes to all existing type-specific sub-indexes.

src/brainy.ts

@@ -2691,6 +2691,24 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       }
     }
 
+    // OPTIMIZATION: Defer HNSW persistence during batch insert.
+    // Without this, each add() triggers ~16-20 neighbor saveHNSWData calls
+    // (each a read→gzip→atomic-write cycle). For 450 items that's ~8,100
+    // individual storage writes. Deferred mode collects dirty node IDs and
+    // flushes once at the end — deduplicating repeated neighbor updates.
+    const index = this.index as any
+    const prevPersistMode = typeof index.getPersistMode === 'function'
+      ? index.getPersistMode()
+      : null
+    const canDefer = prevPersistMode === 'immediate'
+      && typeof index.setPersistMode === 'function'
+      && typeof index.flush === 'function'
+
+    if (canDefer) {
+      index.setPersistMode('deferred')
+    }
+
+    try {
       // Process in batches
       for (let i = 0; i < params.items.length; i += batchSize) {
         const chunk = params.items.slice(i, i + batchSize)
@@ -2742,6 +2760,13 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           lastBatchTime = Date.now()
         }
       }
+    } finally {
+      // Restore persist mode and flush all dirty nodes in one pass
+      if (canDefer) {
+        await index.flush()
+        index.setPersistMode(prevPersistMode)
+      }
+    }
 
     result.duration = Date.now() - startTime
     return result

src/hnsw/typeAwareHNSWIndex.ts

@@ -163,6 +163,19 @@ export class TypeAwareHNSWIndex {
     return this.persistMode
   }
 
+  /**
+   * Switch persist mode at runtime. Propagates to all existing type-specific indexes.
+   * Used by addMany() to defer persistence during batch operations.
+   */
+  public setPersistMode(mode: 'immediate' | 'deferred'): void {
+    this.persistMode = mode
+    for (const index of this.indexes.values()) {
+      if (typeof (index as any).setPersistMode === 'function') {
+        (index as any).setPersistMode(mode)
+      }
+    }
+  }
+
   /**
    * Get or create HNSW index for a specific type (lazy initialization)
    *