feat(04-01): register GravityFlipWand interaction + item JSON (WAND-01, WAND-02)

- GravityFlipWandInteraction extends SimpleInstantInteraction; Primary/Secondary
  routed through firstRun(), writes to WandSelectionStore via volatile bindStore()
- Items/gravityflip_wand.json: Utility item (Icon Torch_Fire) whose Primary and
  Secondary Interactions reference Type=GravityFlipWand
- GravityFlipPlugin.setup(): constructs WandSelectionStore, injects it into the
  interaction class, then registers via getCodecRegistry(Interaction.CODEC)
  (pattern from InstancesPlugin:158 / ExitInstanceInteraction)
- Expose wandSelections() getter for Phase 04-02+ commands

src/main/java/com/mythlane/gravityflip/GravityFlipPlugin.java

@@ -15,6 +15,9 @@ import com.mythlane.gravityflip.region.GravityFlipRegion;
 import com.mythlane.gravityflip.region.RegionRegistry;
 import com.mythlane.gravityflip.tick.RegionTickLoop;
 import com.mythlane.gravityflip.viz.RegionVisualizer;
+import com.mythlane.gravityflip.wand.GravityFlipWandInteraction;
+import com.mythlane.gravityflip.wand.WandSelectionStore;
+import com.hypixel.hytale.server.core.modules.interaction.interaction.config.Interaction;
 
 import java.util.concurrent.ScheduledFuture;
 import java.util.logging.Level;
@@ -46,6 +49,7 @@ public class GravityFlipPlugin extends JavaPlugin {
     private GravityApplier gravityApplier;
     private FallDamageGuard fallDamageGuard;
     private RegionVisualizer regionVisualizer;
+    private WandSelectionStore wandSelectionStore;
 
     public GravityFlipPlugin(JavaPluginInit init) {
         super(init);
@@ -69,6 +73,18 @@ public class GravityFlipPlugin extends JavaPlugin {
         getEntityStoreRegistry().registerSystem(new FallDamageSuppressorSystem(
                 fallDamageGuard,
                 th -> getLogger().at(Level.WARNING).withCause(th).log("fallDamageSuppressor handle failed")));
+
+        // Plan 04-01 : Gravity Flip wand.
+        // Interaction binding pattern per 04-00 SPIKE-RESULT (Finding 3) — same shape as
+        // InstancesPlugin.java:158 / ExitInstanceInteraction. The JSON Item at
+        // src/main/resources/Items/gravityflip_wand.json references this Type in
+        // Interactions.Primary / Interactions.Secondary.
+        this.wandSelectionStore = new WandSelectionStore();
+        GravityFlipWandInteraction.bindStore(this.wandSelectionStore);
+        getCodecRegistry(Interaction.CODEC).register(
+                "GravityFlipWand",
+                GravityFlipWandInteraction.class,
+                GravityFlipWandInteraction.CODEC);
     }
 
     @Override
@@ -151,6 +167,14 @@ public class GravityFlipPlugin extends JavaPlugin {
     /** Exposed for Phase 3 (gravity physics) and Phase 4 (commands). */
     public RegionRegistry regions() { return registry; }
 
+    /**
+     * Per-player wand selection store. Populated by
+     * {@link GravityFlipWandInteraction}; consumed by
+     * {@code /gravityflip define} (Phase 04-02+).
+     * <p>Returns {@code null} until {@link #setup()} has run.
+     */
+    public WandSelectionStore wandSelections() { return wandSelectionStore; }
+
     /**
      * Accessor for the region config holder. <strong>SAVE CONTRACT:</strong> any
      * caller that mutates {@code configHolder().get().getRegions()} MUST call