Add R/L/D protocol extensions for RSSI sky scanning

Extend the rotctld wire protocol with three new commands that enable
signal strength measurement through the Carryout G2's DVB subsystem:

- R [n]: Read RSSI (handles motor→dvb→rssi→motor menu dance internally)
- L: Enable LNA for signal reception (one-time pre-scan setup)
- D: Discover capabilities (returns CAPS:rssi,lna for G2, empty for others)

Non-G2 protocols return RPRT -6 (not available) for R and L commands.
The menu state invariant is maintained after every operation so P commands
continue to work between RSSI reads.

src/travler_rotor/antenna.py

@@ -50,6 +50,11 @@ class TravlerAntenna:
     def config(self) -> AntennaConfig:
         return self._config
 
+    @property
+    def protocol(self) -> FirmwareProtocol:
+        """Access the underlying firmware protocol (for capability checks)."""
+        return self._protocol
+
     @property
     def is_connected(self) -> bool:
         return self._protocol.is_connected

src/travler_rotor/rotctld.py

@@ -8,6 +8,12 @@ Hamlib clients) use for AZ/EL rotor control:
     S   — stop / disconnect
     _   — get model name
     q   — quit connection
+
+Extended commands for sky-scan integration (CarryoutG2 only):
+
+    R   — read RSSI signal strength ("R [iterations]")
+    L   — enable LNA for signal reception
+    D   — discover supported protocol extensions
 """
 
 from __future__ import annotations
@@ -16,6 +22,7 @@ import logging
 import socket
 
 from travler_rotor.antenna import TravlerAntenna
+from travler_rotor.protocol import CarryoutG2Protocol
 
 logger = logging.getLogger(__name__)
 
@@ -97,6 +104,12 @@ class RotctldServer:
                 self._handle_model_name(conn)
             elif cmd == "q":
                 break
+            elif cmd == "R":
+                self._handle_read_rssi(conn, cmd_parts)
+            elif cmd == "L":
+                self._handle_enable_lna(conn)
+            elif cmd == "D":
+                self._handle_capabilities(conn)
             else:
                 logger.warning("Unknown command: %s", cmd)
                 conn.sendall(b"RPRT -1\n")
@@ -133,3 +146,61 @@ class RotctldServer:
     def _handle_model_name(self, conn: socket.socket) -> None:
         """Respond to '_' — return model identification string."""
         conn.sendall(f"{MODEL_NAME}\n".encode())
+
+    def _handle_read_rssi(self, conn: socket.socket, parts: list[str]) -> None:
+        """Respond to 'R [n]' — read RSSI signal strength.
+
+        Requires CarryoutG2Protocol. Handles the DVB menu switching internally:
+        motor menu -> quit -> dvb menu -> rssi -> quit dvb -> motor menu.
+        Non-G2 rotors return RPRT -6 (not available).
+        """
+        if not isinstance(self._antenna.protocol, CarryoutG2Protocol):
+            conn.sendall(b"RPRT -6\n")
+            return
+
+        try:
+            protocol: CarryoutG2Protocol = self._antenna.protocol  # type: ignore[assignment]
+            iterations = 10
+            if len(parts) > 1:
+                iterations = int(parts[1])
+
+            protocol.quit_submenu()
+            protocol.enter_dvb_menu()
+            reading = protocol.get_rssi(iterations)
+            protocol.quit_submenu()
+            protocol.enter_motor_menu()
+
+            response = f"{reading.reads}\n{reading.average}\n{reading.current}\n"
+            conn.sendall(response.encode("utf-8"))
+        except Exception:
+            logger.exception("Failed to read RSSI")
+            conn.sendall(b"RPRT -1\n")
+
+    def _handle_enable_lna(self, conn: socket.socket) -> None:
+        """Respond to 'L' — enable LNA for signal reception.
+
+        One-time setup before scanning. Requires CarryoutG2Protocol.
+        """
+        if not isinstance(self._antenna.protocol, CarryoutG2Protocol):
+            conn.sendall(b"RPRT -6\n")
+            return
+
+        try:
+            protocol: CarryoutG2Protocol = self._antenna.protocol  # type: ignore[assignment]
+            protocol.quit_submenu()
+            protocol.enter_dvb_menu()
+            protocol.enable_lna()
+            protocol.quit_submenu()
+            protocol.enter_motor_menu()
+
+            conn.sendall(b"RPRT 0\n")
+        except Exception:
+            logger.exception("Failed to enable LNA")
+            conn.sendall(b"RPRT -1\n")
+
+    def _handle_capabilities(self, conn: socket.socket) -> None:
+        """Respond to 'D' — discover supported protocol extensions."""
+        if isinstance(self._antenna.protocol, CarryoutG2Protocol):
+            conn.sendall(b"CAPS:rssi,lna\n")
+        else:
+            conn.sendall(b"CAPS:\n")