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

src/main/resources/locales/ru.yml

@@ -71,6 +71,24 @@ play:
       name: "&#00D4FF&l%s"
       lore: ""
 
+  multi:
+    name: "&#FFFFFFМульти"
+
+    duels:
+      item:
+        material: "nether_star"
+        name: "&#FF6464&lДуэли"
+        lore: "&#A0A0A0Гонка до цели!||&#A0A0A0У каждого игрока своя трасса"
+      start:
+        material: "white_banner"
+        name: "&#00D4FF&lДуэли"
+        lore: ""
+      go: "&#55FF55&lВПЕРЁД!"
+      overview: "&#FFD700&l%s &#A0A0A0победил в дуэли! Время: &#FFFFFF%s"
+      place: "&#0072B3#%s &#A0A0A0%s &#404040- &#A0A0A0%s"
+      victory: "&#55FF55&lПОБЕДА!"
+      loss: "&#FF4444&lПОРАЖЕНИЕ"
+
 community:
   name: "&#FFFFFFСообщество"