Raise online play to 16-player lobbies and 8-player games

Lobbies hold 16 players and the sandbox 16 fighters, games up to 8. The arena
gets spawns 5-8 (outer ledges and middle platforms) and the race start eight
places. The sandbox adds eight more spawns, each the open spot farthest from
the rest. Rival cards wrap into rows of four, the lobby's player list goes to
two tiny columns when crowded, and the server keeps frame packets to one
datagram. Formatted with clang-format.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/protocol.h

@@ -28,7 +28,7 @@
 #include <type_traits>
 
 constexpr int PROTOCOL_VERSION = 3;
-constexpr int LOBBY_MEMBERS_MAX = 4;
+constexpr int LOBBY_MEMBERS_MAX = 16;
 constexpr int LOBBY_CODE_LENGTH = 4;
 // Letters only, without I and O, so codes are easy to read out and type.
 constexpr const char LOBBY_CODE_LETTERS[] = "ABCDEFGHJKLMNPQRSTUVWXYZ";