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/CMakeLists.txt

@@ -3,4 +3,9 @@
 # This software is released under the MIT License.
 # https://opensource.org/licenses/MIT
 
+include(dusktest)
+
+dusktest(test_network.c)
+
 add_subdirectory(http)
+add_subdirectory(socket)