Add UDP socket client/server multiplayer protocol

Implements the multiplayer transport layer from ROADMAP.md: UUID-based
client identity, a reliable-packet channel over UDP, handshake/ack/
ping/disconnect/player-joined/player-left/player-state packet types,
and split online (networked) vs offline (in-process singleplayer)
server modes wired into the engine's update/dispose loop.

test/network/socket/CMakeLists.txt

@@ -0,0 +1,9 @@
+# Copyright (c) 2026 Dominic Masters
+#
+# This software is released under the MIT License.
+# https://opensource.org/licenses/MIT
+
+include(dusktest)
+
+dusktest(test_networksocket.c)
+dusktest(test_serveroffline.c)