fix: Move lines=[] above lock in _read_cb() (Issue #683)

UnboundLocalError when _ser is None — lines was only assigned inside
the else branch. Move initialisation to function scope so the for-loop
outside the lock always has a valid list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

jetson/ros2_ws/src/saltybot_bridge/saltybot_bridge/saltybot_cmd_node.py

@@ -154,12 +154,12 @@ class SaltybotCmdNode(Node):
     # ── RX — telemetry read ───────────────────────────────────────────────────
 
     def _read_cb(self):
+        lines = []
         with self._ser_lock:
             if self._ser is None or not self._ser.is_open:
                 pass
             else:
                 try:
-                    lines = []
                     while self._ser.in_waiting:
                         raw = self._ser.readline()
                         if raw: