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/c/udps_client.h

@@ -52,7 +52,8 @@ enum {
     UDPS_PKT_CONFIG     = 1, /**< Server -> client: signal metadata. */
     UDPS_PKT_ACK        = 2, /**< Client -> server: keepalive. */
     UDPS_PKT_CONNECT    = 3, /**< Client -> server: open a session. */
-    UDPS_PKT_DISCONNECT = 4  /**< Either direction: close a session. */
+    UDPS_PKT_DISCONNECT = 4, /**< Either direction: close a session. */
+    UDPS_PKT_SUBSCRIBE  = 5  /**< Client to TCP stream service only: payload = stream name(s), comma separated; server frames carry the stream index in fragmentIdx. */
 };
 
 /** Sample type codes (udps_signal_t::type_code). */

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

Common/UDP/UDPSProtocol.h

@@ -61,6 +61,14 @@ static const uint8 UDPS_TYPE_CONFIG     = 1u; ///< Server → Client: signal con
 static const uint8 UDPS_TYPE_ACK        = 2u; ///< Client → Server: ack counter
 static const uint8 UDPS_TYPE_CONNECT    = 3u; ///< Client → Server: connect request
 static const uint8 UDPS_TYPE_DISCONNECT = 4u; ///< Client → Server: disconnect
+static const uint8 UDPS_TYPE_SUBSCRIBE  = 5u; ///< Client → Server (TCP stream service only): payload = stream name(s), comma separated
+
+/** TCP stream service: most streams one connection may multiplex. On such a
+ *  connection every server frame carries the stream's position in the SUBSCRIBE
+ *  list in the header's fragmentIdx field (totalFragments stays 1). */
+static const uint32 UDPS_TCP_MAX_STREAMS = 64u;
+/** TCP stream service: longest SUBSCRIBE payload (comma separated names, each <= 255). */
+static const uint32 UDPS_TCP_MAX_SUBSCRIBE_PAYLOAD = 4096u;
 
 /*---------------------------------------------------------------------------*/
 /*                        Wire packet header (17 bytes)                      */