feat: add Duels mode (MultiMode, generator-per-player)
- duels/: DuelsMode (MultiMode), DuelsGenerator (owner: per-player
SingleDuelsGenerator map, allowJoining, goal), SingleDuelsGenerator
(getPlayers()=List.of(own player) -> independent course/score; score
reaches goal -> win), DuelsCountdown (10s countdown + race start)
- win(): sorted scoreboard, victory/loss titles, return to default after 10s;
leave() only removePlayer (no Coop recursion)
- config modes.duels.{enabled,max-players,goal,island-distance}, locales
play.multi.duels.* (ru/en), registered in Modes/PluginBootstrap
- checkstyle 0/0, PMD 0, build green