Fix Noise session persistence during peer ID rotation

- Add session migration when peer IDs rotate
- Sessions now follow peers across ID changes via fingerprint
- Add migratePeerSession to NoiseEncryptionService
- Add migrateSession to NoiseSessionManager
- Integrate migration in BluetoothMeshService updatePeerBinding

This fixes the issue where established Noise sessions were lost
when peer IDs rotated, causing "No Noise session" errors and
requiring re-handshake.

bitchat/Services/NoiseEncryptionService.swift

@@ -279,6 +279,24 @@ class NoiseEncryptionService {
         }
     }
     
+    /// Migrate session when peer ID changes
+    func migratePeerSession(from oldPeerID: String, to newPeerID: String, fingerprint: String) {
+        // First update the fingerprint mappings
+        serviceQueue.sync(flags: .barrier) {
+            // Remove old mapping
+            if let oldFingerprint = peerFingerprints[oldPeerID], oldFingerprint == fingerprint {
+                peerFingerprints.removeValue(forKey: oldPeerID)
+            }
+            
+            // Add new mapping
+            peerFingerprints[newPeerID] = fingerprint
+            fingerprintToPeerID[fingerprint] = newPeerID
+        }
+        
+        // Migrate the session in session manager
+        sessionManager.migrateSession(from: oldPeerID, to: newPeerID)
+    }
+    
     // MARK: - Private Helpers
     
     private func handleSessionEstablished(peerID: String, remoteStaticKey: Curve25519.KeyAgreement.PublicKey) {