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.

ROADMAP.md

@@ -33,8 +33,9 @@ for a memory/CPU optimization survey specifically targeting the PSP build
 9. Build the socket server and client implementation, including
    handlers for the different packet types.
 10. Add a dedicated multiplayer entity type, `clientplayer`, alongside
-    the existing `npc` and `player` types. Limit to 8 (defined
-    constant) for now.
+    the existing `npc` and `player` types. Limit to 16 (defined
+    constant, see SERVER_CLIENT_COUNT_MAX/CLIENT_COUNT_MAX in
+    src/dusk/network/socket/) for now.
 11. Send and receive `clientplayer` position over the network.
 12. Create a UI menu for creating a server and joining a server. For
     now, join IPs are hard-coded (testing against a fixed IP of