Add DASP device discovery to the connect dialog

Send DISCOVER broadcast (type 0) to 255.255.255.255 and the Sedona
multicast address 239.255.18.76 to find devices on the network.
Responses are listed in a Treeview showing IP and platform ID.
Selecting a device fills the Host field; double-click connects.
Wait time is configurable (default 10s, range 1-60s).

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

dasp.py

@@ -945,8 +945,55 @@ class DaspSocket:
                     continue
                 break
 
+    def discover(self, port=DEFAULT_PORT, wait_time=3):
+        """Send DASP DISCOVER broadcast and collect responding devices.
+
+        Returns a list of (ip_address, platform_id) tuples.
+        """
+        self._discovered = []
+        self._discover_active = True
+
+        msg = DaspMsg()
+        msg.msg_type = DISCOVER
+        msg.session_id = 0xFFFF
+        msg.seq_num = 0xFFFF
+        msg.include_message_len = False
+        data = msg.encode()
+
+        with self._send_lock:
+            try:
+                self._sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
+            except OSError:
+                pass
+            try:
+                self._sock.sendto(data, ("255.255.255.255", port))
+            except OSError:
+                pass
+            # also try the multicast address used by the Sedona spec
+            try:
+                self._sock.sendto(data, ("239.255.18.76", port))
+            except OSError:
+                pass
+
+        time.sleep(wait_time)
+        self._discover_active = False
+
+        seen = set()
+        result = []
+        for addr, pid in self._discovered:
+            if addr not in seen:
+                seen.add(addr)
+                result.append((addr, pid or ""))
+        return result
+
     def _dispatch(self, msg, addr, raw=None):
         _trace("R", msg, raw)
+
+        # Handle DISCOVER responses (from devices replying to our broadcast)
+        if msg.msg_type == DISCOVER and getattr(self, '_discover_active', False):
+            self._discovered.append((addr[0], msg.platform_id))
+            return
+
         session_id = msg.session_id
         with self._sessions_lock:
             session = self._sessions.get(session_id)