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/menu/core/Menus.java

@@ -3,6 +3,7 @@ package dev.loki.loparkour.menu.core;
 import dev.loki.loparkour.menu.community.CommunityMenu;
 import dev.loki.loparkour.menu.community.LeaderboardsMenu;
 import dev.loki.loparkour.menu.community.SingleLeaderboardMenu;
+import dev.loki.loparkour.menu.lobby.DuelsMenu;
 import dev.loki.loparkour.menu.lobby.LobbyMenu;
 import dev.loki.loparkour.menu.lobby.PlayerManagementMenu;
 import dev.loki.loparkour.menu.play.PlayMenu;
@@ -37,6 +38,7 @@ public final class Menus {
     public static ParkourSettingsMenu PARKOUR_SETTINGS = new ParkourSettingsMenu();
 
     // lobby
+    public static DuelsMenu DUELS = new DuelsMenu();
     public static LobbyMenu LOBBY = new LobbyMenu();
     public static PlayerManagementMenu PLAYER_MANAGEMENT = new PlayerManagementMenu();
 

src/main/java/dev/loki/loparkour/menu/lobby/DuelsMenu.java

@@ -0,0 +1,123 @@
+package dev.loki.loparkour.menu.lobby;
+
+import dev.loki.loparkour.config.locale.Locales;
+import dev.loki.loparkour.duels.DuelsGenerator;
+import dev.loki.loparkour.menu.core.LPMenu;
+import dev.loki.loparkour.mode.base.Modes;
+import dev.loki.loparkour.player.core.ParkourUser;
+import dev.loki.loparkour.session.core.Session;
+import dev.loki.loparkour.util.text.ColorUtil;
+import dev.loki.loparkour.world.core.Divider;
+import dev.lolib.gui.InventoryGUI;
+import org.bukkit.Material;
+import org.bukkit.entity.Player;
+import org.bukkit.inventory.ItemStack;
+import org.bukkit.inventory.meta.ItemMeta;
+import org.bukkit.inventory.meta.SkullMeta;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Lists open duels sessions and lets the player join one.
+ */
+public class DuelsMenu extends LPMenu {
+
+    private static final int ROWS = 3;
+    private static final int ITEMS_PER_PAGE = 7;
+    private static final int FIRST_SLOT = 10;
+    private static final int PREV_SLOT = 0;
+    private static final int NEXT_SLOT = 8;
+    private static final int SLOT_EMPTY = 13;
+    private static final int SLOT_CLOSE = 22;
+
+    private int currentPage = 0;
+
+    @Override
+    public void open(@NotNull Player player) {
+        open(player, 0);
+    }
+
+    public void open(@NotNull Player player, int page) {
+        this.currentPage = page;
+        final String locale = locale(player);
+        final String title = Locales.getString(locale, "play.multi.duels.name");
+
+        final List<Session> sessions = collectOpenSessions();
+
+        final int totalPages = (int) Math.ceil((double) sessions.size() / ITEMS_PER_PAGE);
+        final int startIndex = currentPage * ITEMS_PER_PAGE;
+        final int endIndex = Math.min(startIndex + ITEMS_PER_PAGE, sessions.size());
+
+        InventoryGUI gui = baseGui(title, ROWS);
+
+        if (sessions.isEmpty()) {
+            gui = gui.setItem(SLOT_EMPTY, localeItem(player, "play.multi.duels.menu_empty"));
+        } else {
+            for (int i = startIndex; i < endIndex; i++) {
+                final Session session = sessions.get(i);
+                final int slot = FIRST_SLOT + (i - startIndex);
+                gui = gui.setItem(slot, sessionItem(player, session), e -> {
+                    Modes.DUELS.join(player, session);
+                    player.closeInventory();
+                });
+            }
+        }
+
+        if (currentPage > 0) {
+            gui = gui.setItem(PREV_SLOT, backItem(player), e -> open(player, currentPage - 1));
+        }
+
+        if (currentPage < totalPages - 1) {
+            gui = gui.setItem(NEXT_SLOT, nextItem(player), e -> open(player, currentPage + 1));
+        }
+
+        gui.setItem(SLOT_CLOSE, closeButton(player), e -> player.closeInventory())
+                .open(player);
+    }
+
+    private List<Session> collectOpenSessions() {
+        final int maxPlayers = Modes.DUELS.getMaxPlayers();
+        final List<Session> sessions = new ArrayList<>();
+        for (final Session session : Divider.SECTIONS.keySet()) {
+            if (isOpenForJoining(session, maxPlayers)) {
+                sessions.add(session);
+            }
+        }
+        return sessions;
+    }
+
+    private boolean isOpenForJoining(Session session, int maxPlayers) {
+        return session.generator instanceof DuelsGenerator generator
+                && generator.allowJoining
+                && session.getPlayers().size() < maxPlayers;
+    }
+
+    private ItemStack sessionItem(@NotNull Player player, @NotNull Session session) {
+        final DuelsGenerator generator = (DuelsGenerator) session.generator;
+        final ItemStack item = new ItemStack(Material.PLAYER_HEAD);
+        applySkullOwner(player, item, generator.owner);
+
+        final ItemMeta meta = item.getItemMeta();
+        if (meta != null) {
+            meta.setDisplayName(ColorUtil.color(generator.owner.getName()));
+            item.setItemMeta(meta);
+        }
+        return item;
+    }
+
+    private void applySkullOwner(@NotNull Player player, @NotNull ItemStack skull, @NotNull ParkourUser owner) {
+        if (ParkourUser.isBedrockPlayer(player)) {
+            return;
+        }
+        if (owner.getName() == null || owner.getName().startsWith(".")) {
+            return;
+        }
+        final SkullMeta meta = (SkullMeta) skull.getItemMeta();
+        if (meta != null) {
+            meta.setOwningPlayer(owner.player);
+            skull.setItemMeta(meta);
+        }
+    }
+}