Add TCP control + UDP multicast mode to UDPStreamer

DataSource (C++):
- New optional config: MulticastGroup (e.g. "239.0.0.1") and DataPort
  (default Port+1); when set, enables multicast mode; absent = unicast
- Control plane: BasicTCPSocket listener on Port; CONNECT received over
  TCP triggers HandleTCPConnect() which sends CONFIG via the same TCP
  connection
- Data plane: BasicUDPSocket aimed at MulticastGroup:DataPort; all DATA
  fragments sent via dataSocket.Write() so any joined client receives them
- useMulticast is the single branch point; every unicast code path is
  unchanged when MulticastGroup is absent
- New methods: HandleTCPConnect(), IsMulticast()
- 5 new unit tests (ports 44710-44729); all 38 tests passing

WebUI hub (Go):
- SourceConfig gains MulticastGroup and DataPort fields (JSON, optional)
- UDPClient gains multicastGroup/dataPort; Run() routes to new
  runMulticastSession() when multicastGroup is non-empty
- runMulticastSession(): TCP dial for CONNECT/CONFIG, net.ListenMulticastUDP
  for DATA, background goroutine watches TCP for DISCONNECT/close
- All existing sm.Add() call sites updated (unicast callers pass "", 0)

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

Client/WebUI/hub.go

@@ -474,7 +474,7 @@ func (h *Hub) Run() {
 
 			case "wsAddSource":
 				if h.sm != nil {
-					go func(label, addr string) { h.sm.Add(label, addr) }(cmd.label, cmd.addr)
+					go func(label, addr string) { h.sm.Add(label, addr, "", 0) }(cmd.label, cmd.addr)
 				}
 
 			case "wsRemoveSource":