fix(ble): Add get_peer_mtu method to LinuxBluetoothDriver

## Problem

The identity handshake handler called `self.driver.get_peer_mtu(address)`,
but this method didn't exist, causing:

```
[Error] BLEInterface[BLE Interface] failed to process identity handshake from dev:B8:27:EB:A8:A7:22: 'LinuxBluetoothDriver' object has no attribute 'get_peer_mtu'
```

## Solution

Added `get_peer_mtu(address)` method to LinuxBluetoothDriver that:

1. Checks central connections (self._peers) for MTU when we're the central
2. Checks peripheral connections (gatt_server.connected_centrals) for MTU when we're the peripheral
3. Returns None if peer not found in either

This mirrors the existing `get_peer_role()` pattern and provides
thread-safe access to MTU information for both connection types.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/RNS/Interfaces/linux_bluetooth_driver.py

@@ -1088,6 +1088,25 @@ class LinuxBluetoothDriver(BLEDriverInterface):
                 return self._peers[address].connection_type
             return None
 
+    def get_peer_mtu(self, address: str) -> Optional[int]:
+        """Return the negotiated MTU for a peer connection.
+
+        Checks both central connections (we connected to them) and peripheral
+        connections (they connected to us).
+        """
+        # Check central connections (we are central)
+        with self._peers_lock:
+            if address in self._peers:
+                return self._peers[address].mtu
+
+        # Check peripheral connections (we are peripheral, they are central)
+        if self.gatt_server:
+            with self.gatt_server.centrals_lock:
+                if address in self.gatt_server.connected_centrals:
+                    return self.gatt_server.connected_centrals[address].get("mtu")
+
+        return None
+
     def set_service_discovery_delay(self, seconds: float):
         """Set delay between connection and service discovery."""
         self.service_discovery_delay = seconds