feat(04-02): add GravityFlipCommand root + wand subcommand (CMD-01)

- GravityFlipCommand extends AbstractCommandCollection (pattern: TeleportCommand)
- GravityFlipWandSubCommand extends AbstractPlayerCommand (pattern: GiveCommand)
- resolves 'gravityflip_wand' via Item.getAssetMap().getAsset, giveItem, chat feedback
- null-safe on asset miss + inventory-full remainder check

src/main/java/com/mythlane/gravityflip/command/GravityFlipCommand.java

@@ -0,0 +1,30 @@
+package com.mythlane.gravityflip.command;
+
+import com.hypixel.hytale.server.core.command.system.basecommands.AbstractCommandCollection;
+import com.mythlane.gravityflip.GravityFlipPlugin;
+
+/**
+ * Root command {@code /gravityflip}. Aggregates all Gravity Flip sub-commands.
+ *
+ * <p>Extends {@link AbstractCommandCollection} — pattern sourced from
+ * {@code builtin/teleport/commands/teleport/TeleportCommand} and
+ * {@code modules/debug/commands/DebugCommand}. When invoked without a
+ * sub-command, the base class emits a usage message listing registered
+ * sub-commands (no extra work required here).
+ *
+ * <p>The plugin reference is stored for future sub-commands (04-03 define,
+ * 04-04 list/delete/toggle/tp) that need {@code plugin.wandSelections()} or
+ * {@code plugin.configHolder()}.
+ */
+public final class GravityFlipCommand extends AbstractCommandCollection {
+
+    public GravityFlipCommand(GravityFlipPlugin plugin) {
+        super("gravityflip", "Commandes de gestion des zones Gravity Flip");
+        this.addSubCommand(new GravityFlipWandSubCommand());
+        // 04-03 will addSubCommand(new GravityFlipDefineSubCommand(plugin));
+        // 04-04 will addSubCommand(new GravityFlipListSubCommand(plugin));
+        //        addSubCommand(new GravityFlipDeleteSubCommand(plugin));
+        //        addSubCommand(new GravityFlipToggleSubCommand(plugin));
+        //        addSubCommand(new GravityFlipTpSubCommand(plugin));
+    }
+}

src/main/java/com/mythlane/gravityflip/command/GravityFlipWandSubCommand.java

@@ -0,0 +1,68 @@
+package com.mythlane.gravityflip.command;
+
+import com.hypixel.hytale.component.Ref;
+import com.hypixel.hytale.component.Store;
+import com.hypixel.hytale.server.core.Message;
+import com.hypixel.hytale.server.core.asset.type.item.config.Item;
+import com.hypixel.hytale.server.core.command.system.CommandContext;
+import com.hypixel.hytale.server.core.command.system.basecommands.AbstractPlayerCommand;
+import com.hypixel.hytale.server.core.entity.entities.Player;
+import com.hypixel.hytale.server.core.inventory.ItemStack;
+import com.hypixel.hytale.server.core.inventory.transaction.ItemStackTransaction;
+import com.hypixel.hytale.server.core.universe.PlayerRef;
+import com.hypixel.hytale.server.core.universe.world.World;
+import com.hypixel.hytale.server.core.universe.world.storage.EntityStore;
+import javax.annotation.Nonnull;
+
+/**
+ * {@code /gravityflip wand} — donne un Gravity Flip Wand au joueur appelant.
+ *
+ * <p>Pattern copié ligne-à-ligne sur
+ * {@code com.hypixel.hytale.server.core.command.commands.player.inventory.GiveCommand}
+ * (cf. GiveCommand.java:47-76). La seule différence : l'item est résolu par
+ * constante ({@link #WAND_ITEM_ID}) au lieu d'un {@code RequiredArg<Item>}.
+ *
+ * <p>L'item {@code gravityflip_wand} est enregistré via le JSON bundle
+ * {@code src/main/resources/Items/gravityflip_wand.json} (Phase 04-01).
+ * Si l'asset n'est pas chargé (JSON absent / pas picked-up par l'AssetStore
+ * core), le lookup renvoie {@code null} et la commande envoie un message
+ * d'erreur clair au lieu de crasher (T-04-02 scope).
+ */
+public final class GravityFlipWandSubCommand extends AbstractPlayerCommand {
+
+    /** ItemID du wand défini dans {@code Items/gravityflip_wand.json} (Phase 04-01). */
+    private static final String WAND_ITEM_ID = "gravityflip_wand";
+
+    public GravityFlipWandSubCommand() {
+        super("wand", "Obtenir un Gravity Flip Wand");
+    }
+
+    @Override
+    protected void execute(@Nonnull CommandContext context,
+                           @Nonnull Store<EntityStore> store,
+                           @Nonnull Ref<EntityStore> ref,
+                           @Nonnull PlayerRef playerRef,
+                           @Nonnull World world) {
+        Item item = Item.getAssetMap().getAsset(WAND_ITEM_ID);
+        if (item == null) {
+            context.sendMessage(Message.raw(
+                    "[gravityflip] Item '" + WAND_ITEM_ID
+                            + "' introuvable — asset pas chargé ?"));
+            return;
+        }
+        Player playerComponent = store.getComponent(ref, Player.getComponentType());
+        if (playerComponent == null) {
+            context.sendMessage(Message.raw("[gravityflip] Player component manquant."));
+            return;
+        }
+        ItemStack stack = new ItemStack(item.getId(), 1, null);
+        ItemStackTransaction transaction = playerComponent.giveItem(stack, ref, store);
+        ItemStack remainder = transaction.getRemainder();
+        if (remainder == null || remainder.isEmpty()) {
+            context.sendMessage(Message.raw("[gravityflip] Wand ajouté à ton inventaire."));
+        } else {
+            context.sendMessage(Message.raw(
+                    "[gravityflip] Inventaire plein — impossible d'ajouter le wand."));
+        }
+    }
+}