TPG.Voting.CastMapVote

Record (or change) ply's vote for a map-vote candidate and broadcast the new tally. mapIndex is whatever the client's tpg_votemap concommand sent -- an unchecked number from the player. It is safe here only because the lookup is bounds-checked: TPG.State.voting.maps[mapIndex] returns nil for anything out of range and the function returns immediately, so a forged index can neither crash the server nor vote for a map that isn't a candidate. No-ops entirely while no vote is active.