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/sources.go

@@ -11,16 +11,20 @@ import (
 
 // SourceConfig is the serialisable description of one data source (for file save/load).
 type SourceConfig struct {
-	Label string `json:"label"`
-	Addr  string `json:"addr"`
+	Label          string `json:"label"`
+	Addr           string `json:"addr"`
+	MulticastGroup string `json:"multicastGroup,omitempty"` // "" = unicast mode
+	DataPort       int    `json:"dataPort,omitempty"`        // 0 = Addr port+1
 }
 
 // managedSource is the SourceManager's view of one running source.
 type managedSource struct {
-	id     string
-	label  string
-	addr   string
-	client *UDPClient
+	id             string
+	label          string
+	addr           string
+	multicastGroup string
+	dataPort       int
+	client         *UDPClient
 }
 
 // SourceManager owns the lifecycle of all active data sources.
@@ -48,14 +52,20 @@ func (sm *SourceManager) genID() string {
 }
 
 // Add creates a new source, registers it in the hub, and starts connecting.
+// multicastGroup: multicast IP (e.g. "239.0.0.1") or "" for unicast mode.
+// dataPort: UDP data port in multicast mode; 0 = Addr port+1.
 // Returns the source ID.
-func (sm *SourceManager) Add(label, addr string) string {
+func (sm *SourceManager) Add(label, addr, multicastGroup string, dataPort int) string {
 	if label == "" {
 		label = addr
 	}
 	id := sm.genID()
-	c := NewUDPClient(addr, id, sm.hub)
-	ms := &managedSource{id: id, label: label, addr: addr, client: c}
+	c := NewUDPClient(addr, id, sm.hub, multicastGroup, dataPort)
+	ms := &managedSource{
+		id: id, label: label, addr: addr,
+		multicastGroup: multicastGroup, dataPort: dataPort,
+		client: c,
+	}
 
 	sm.mu.Lock()
 	sm.sources[id] = ms
@@ -90,7 +100,12 @@ func (sm *SourceManager) Save() error {
 	sm.mu.RLock()
 	cfgs := make([]SourceConfig, 0, len(sm.sources))
 	for _, ms := range sm.sources {
-		cfgs = append(cfgs, SourceConfig{Label: ms.label, Addr: ms.addr})
+		cfgs = append(cfgs, SourceConfig{
+			Label:          ms.label,
+			Addr:           ms.addr,
+			MulticastGroup: ms.multicastGroup,
+			DataPort:       ms.dataPort,
+		})
 	}
 	sm.mu.RUnlock()
 
@@ -113,7 +128,7 @@ func (sm *SourceManager) Load(path string) error {
 	}
 	sm.filePath = path
 	for _, cfg := range cfgs {
-		sm.Add(cfg.Label, cfg.Addr)
+		sm.Add(cfg.Label, cfg.Addr, cfg.MulticastGroup, cfg.DataPort)
 	}
 	return nil
 }