add dbg info helper

Author: NiloCK

packages/db/src/study/services/CardHydrationService.ts

@@ -79,6 +79,13 @@ export class CardHydrationService<TView = unknown> {
     return this.hydratedQ.length;
   }
 
+  /**
+   * Get current failed card cache size.
+   */
+  public get failedCacheSize(): number {
+    return this.failedCardCache.size;
+  }
+
   /**
    * Fill the hydrated queue up to BUFFER_SIZE with pre-fetched cards.
    */