feat: Add connection handshake to trigger peripheral callbacks

Sends empty WRITE to RX characteristic immediately after connection
to guarantee remote side's on_central_connected callback fires.

Problem: Peripheral callback triggered by WRITE events, not connections.
When central connects and only READs (Identity characteristic), the
peripheral's on_central_connected never fires, preventing peer interface
spawning on the peripheral side.

Solution: After reading Identity, write empty bytes to RX characteristic.
This triggers the WRITE callback which calls _handle_central_connected(),
ensuring bidirectional peer interface spawning.

Benefits:
- Works for Pi-to-Pi (ensures both sides spawn interfaces)
- Works for Android-to-Pi (Pi spawns interface when Android connects)
- Minimal overhead (single empty GATT write)
- Backwards compatible (empty write is harmless)

Implementation:
- Added after Identity read in _connect_to_peer()
- Uses write_gatt_char() with response=True for reliability
- Non-critical failure (logged as warning, doesn't block connection)
- TODO comment for future handshake protocol enhancements

This solves the asymmetric peer spawning issue seen in testing where
only the central side had a peer interface.

Tested: Enables bidirectional data flow for single-direction discoveries.

src/RNS/Interfaces/BLEInterface.py

@@ -1445,6 +1445,16 @@ class BLEInterface(Interface):
                         RNS.log(f"{self} failed to read identity from {peer.name}: {type(e).__name__}: {e}", RNS.LOG_DEBUG)
                         # Continue without identity
 
+                # Send connection handshake to trigger peripheral callback
+                # Write empty bytes to RX characteristic to ensure remote's on_central_connected fires
+                # This guarantees bidirectional peer interface spawning even when only one side discovers
+                # TODO: Consider sending handshake packet with protocol version/capabilities/flags
+                try:
+                    await client.write_gatt_char(self.CHARACTERISTIC_RX_UUID, b'', response=True)
+                    RNS.log(f"{self} sent connection handshake to {peer.name}", RNS.LOG_DEBUG)
+                except Exception as e:
+                    RNS.log(f"{self} handshake write failed (non-critical): {e}", RNS.LOG_WARNING)
+
                 # Get negotiated MTU
                 try:
                     # For BlueZ backend, acquire MTU first to avoid warning