feat: elytra firework boost, obstacle mode, duels menu, tests

- ElytraBoost: one-shot firework boost (v*0.7+dir*0.6, FIREWORKS_SPARK,
  rocket sounds) on entering ascending section + sustained thrust; Particle.
  FIREWORK doesn't exist in spigot-api 1.20.4 -> FIREWORKS_SPARK
- ObstacleGenerator (client-side obstacles: PILLARS/LINES/HOLE_IN_WALL/
  CIRCLE/DIAMOND via ClientBlockChanger.sendNow) + ObstacleMode + locales
- DuelsMenu (lists open duels sessions, click to join) + /parkour duels
  command + tab completion + locales play.multi.duels.{name,menu_empty}
- Tests: ElytraSection/PointType/KnotDirector/SingleDuelsGenerator (15 new)
- checkstyle 0/0 (main+test), PMD 0, 41 tests pass

src/main/java/dev/loki/loparkour/elytra/mode/ObstacleMode.java

@@ -0,0 +1,55 @@
+package dev.loki.loparkour.elytra.mode;
+
+import dev.loki.loparkour.config.locale.Locales;
+import dev.loki.loparkour.elytra.generator.ObstacleGenerator;
+import dev.loki.loparkour.leaderboard.core.Leaderboard;
+import dev.loki.loparkour.mode.base.Mode;
+import dev.loki.loparkour.mode.base.ModeMessages;
+import dev.loki.loparkour.player.core.ParkourPlayer;
+import dev.loki.loparkour.session.core.Session;
+import dev.loki.loparkour.util.item.Item;
+import org.bukkit.entity.Player;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * Elytra mode with client-side obstacles to fly around.
+ */
+public class ObstacleMode implements Mode {
+
+    private final Leaderboard leaderboard = new Leaderboard(getName(), Leaderboard.Sort.SCORE);
+
+    @Override
+    @NotNull
+    public String getName() {
+        return "obstacle";
+    }
+
+    @Override
+    @Nullable
+    public Item getItem(String locale) {
+        return Locales.getItem(locale, "play.elytra.obstacle");
+    }
+
+    @Override
+    @NotNull
+    public Leaderboard getLeaderboard() {
+        return leaderboard;
+    }
+
+    @Override
+    public void create(Player player) {
+        if (!ModeMessages.checkJoiningEnabled(player)) {
+            return;
+        }
+
+        final ParkourPlayer pp = ParkourPlayer.getPlayer(player);
+        if (pp != null && pp.session.generator != null && pp.session.generator.getMode() instanceof ObstacleMode) {
+            return;
+        }
+
+        player.closeInventory();
+
+        Session.create(ObstacleGenerator::new, null, null, player);
+    }
+}