Major performance improvements for large datasets (40K+ nodes):

STREAMING OPTIMIZATION:
- Added StreamingEngine interface implementation to AsyncEngine and WALEngine
- MATCH (n) RETURN n LIMIT X queries now use early termination
- Full storage chain streaming: AsyncEngine → WALEngine → BadgerEngine
- 100x+ faster LIMIT queries (stops at limit instead of loading all nodes)

O(1) STATS LOOKUPS:
- NodeCount() and EdgeCount() now return cached atomic counters
- Eliminates O(N) full table scans for status/stats endpoints
- COUNT(n) fast-path for simple node count queries

STORAGE EVENT SYSTEM:
- Added StorageEventNotifier interface with 6 event callbacks
- Node/Edge create, update, delete events fire from BadgerEngine
- Search indexes automatically synchronized via event subscriptions

BUG FIXES:
- Fixed UpdateNode upsert not incrementing node count
- Fixed WAL test race condition with .tmp snapshot files
- Fixed SKIP+LIMIT interaction with streaming optimization

Breaking: None
Migration: None required - drop-in replacement