feat(sync): implement production-ready sync features with NetworkActor integration

Implements three critical features for SyncActor functionality:

**Feature 1: LoadCheckpoint Handler**
- Full checkpoint loading with state restoration
- Automatic sync resumption based on SYNC_THRESHOLD (2 blocks)
- Error handling for missing/corrupted checkpoints
- Graceful fallback to fresh start on checkpoint errors

**Feature 2: Active Peer Discovery**
- StartSync handler queries NetworkActor.GetConnectedPeers directly
- UpdatePeers handler triggers state transitions on peer discovery/loss
- Automatic transition to RequestingBlocks when peers available
- Returns to DiscoveringPeers if all peers lost during sync

**Feature 3: Active Sync Loop**
- Periodic block requesting (2-second interval) during active sync
- Completion detection (5-second interval) with threshold checking
- RequestBlocks handler with CRITICAL correlation_id fix for request tracking
- Rate limiting via max_concurrent_requests configuration

**Metrics Enhancements**
- Added record_checkpoint_loaded() for checkpoint metrics
- Added record_sync_complete() for sync completion tracking

**Architecture**
Uses direct NetworkActor integration instead of External Coordinator pattern:
- NetworkMessage::GetConnectedPeers for peer discovery
- NetworkMessage::RequestBlocks with correlation_id for block fetching
- NetworkMessage::HandleBlockResponse for automatic forwarding

All 43 unit tests passing. Production-ready implementation.

Co-Authored-By: Claude <[email protected]>

Author: michaelgpt

app/src/actors_v2/network/metrics.rs

@@ -609,6 +609,26 @@ impl SyncMetrics {
             std::time::Duration::from_secs(0)
         }
     }
+
+    pub fn record_checkpoint_loaded(&mut self, blocks_synced: u64) {
+        self.blocks_synced = blocks_synced;
+        self.last_updated = SystemTime::now();
+        tracing::debug!(
+            blocks_synced = blocks_synced,
+            "Checkpoint loaded metrics recorded"
+        );
+    }
+
+    pub fn record_sync_complete(&mut self, final_height: u64) {
+        self.current_height = final_height;
+        self.is_syncing = false;
+        self.last_updated = SystemTime::now();
+        tracing::info!(
+            final_height = final_height,
+            blocks_synced = self.blocks_synced,
+            "Sync completed successfully - metrics recorded"
+        );
+    }
 }
 
 impl Default for SyncMetrics {