fix: UDPSClient uses two-arg Join so multicast receiver lands on the right interface

UDPSClient::ConnectMulticast was calling the single-arg BasicUDPSocket::Join,
which forwards NULL as the local interface and lets the kernel bind to
INADDR_ANY.  On a multi-homed host (or when the server sends on loopback via
Interface = "127.0.0.1") the client joins the wrong interface and silently
receives nothing.

Fix: read the optional Interface key inside the useMulticast block in
UDPSClient::Initialise; in ConnectMulticast call the two-arg
Join(group, interface) when Interface is set, and fall back to the one-arg
call otherwise to preserve the existing INADDR_ANY behaviour for configs that
omit it.

Forward the new optional Interface key through UDPStreamerClient (read from
DataSource config, written into the UDPSClient ConfigurationDatabase only
when non-empty).  Extend the "Joined multicast group" log to report the
interface name or "default".

Regression test TestExecute_MulticastReceivesDataOnInterface: mock TCP
control listener + multicast UDP DATA socket with IP_MULTICAST_IF set to
127.0.0.1, verifying a uint32 value of 424242 reaches DataSource signal
memory.  Confirmed FAILED without the Join fix and PASSED with it.

Suite: 133/133 (was 132/132 before this commit).

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

Source/Components/Interfaces/UDPStream/UDPSClient.h

@@ -116,7 +116,10 @@ public:
      *  - ServerAddr      (char*)  Server IPv4 address. Required.
      *  - Port            (uint16) Server UDP port (unicast) or TCP listen port (multicast). Required.
      *  - MulticastGroup  (char*)  IPv4 multicast address; presence enables multicast mode.
-     *  - Interface       (char*)  Network interface for multicast join (e.g. "lo"). Required when MulticastGroup is set.
+     *  - Interface       (char*)  Local IPv4 dotted-quad address (e.g. "127.0.0.1") of the interface on
+     *                             which to join the multicast group.  Optional; omitting it uses the
+     *                             default-route interface (INADDR_ANY), which silently receives nothing
+     *                             if the server sends on a different interface.
      *  - DataPort        (uint16) UDP multicast data port (defaults to Port+1).
      *  - SilenceTimeout  (float32) Seconds of no data before reconnect. Default 1.0.
      *                      Sub-second values allowed; 0 disables the check.
@@ -201,6 +204,7 @@ private:
     StreamString serverAddr;
     uint16       serverPort;
     StreamString multicastGroup;
+    StreamString multicastInterface;
     uint16       dataPort;
     bool         useMulticast;
     uint64       silenceTimeoutTicks;