feat(tcp): TCPStreamer DataSources, in-process TCPStreamService, stream multiplexing and TCPStreamJoin

Add TCPStreamer/TCPStreamerClient (TCP transport behind the UDPSServerI/UDPSClientI
seam), an optional single in-process TCPStreamService serving many streams over
worker threads, multiplexed SUBSCRIBE (comma list, stream index in fragmentIdx),
and TCPStreamJoin, which merges several streams into one sample-and-hold stream
paced by a master. StreamHub takes a comma list of streams over one connection.
Includes GTests, E2E scenarios s58-s73 and docs.

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

Common/Client/go/udpsprotocol/protocol.go

@@ -18,6 +18,7 @@ const (
 	PktACK        uint8 = 2
 	PktConnect    uint8 = 3
 	PktDisconnect uint8 = 4
+	PktSubscribe  uint8 = 5 // TCP stream service only: payload = stream name(s), comma separated; server frames carry the stream index in fragmentIdx
 
 	HeaderSize   = 17
 	SigDescSize  = 136