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

src/main/java/com/mythlane/gravityflip/wand/GravityFlipWandInteraction.java

@@ -0,0 +1,119 @@
+package com.mythlane.gravityflip.wand;
+
+import com.hypixel.hytale.codec.builder.BuilderCodec;
+import com.hypixel.hytale.component.CommandBuffer;
+import com.hypixel.hytale.component.Ref;
+import com.hypixel.hytale.protocol.BlockPosition;
+import com.hypixel.hytale.protocol.InteractionType;
+import com.hypixel.hytale.server.core.Message;
+import com.hypixel.hytale.server.core.entity.InteractionContext;
+import com.hypixel.hytale.server.core.modules.interaction.interaction.CooldownHandler;
+import com.hypixel.hytale.server.core.modules.interaction.interaction.config.SimpleInstantInteraction;
+import com.hypixel.hytale.server.core.universe.PlayerRef;
+import com.hypixel.hytale.server.core.universe.world.storage.EntityStore;
+
+import javax.annotation.Nonnull;
+import java.util.UUID;
+
+/**
+ * Gravity Flip wand interaction — reads Primary / Secondary clicks and pushes
+ * the targeted block position into a {@link WandSelectionStore} keyed by the
+ * clicker's player UUID.
+ *
+ * <p><b>Binding :</b> registered at {@code setup()} time via
+ * {@code getCodecRegistry(Interaction.CODEC).register("GravityFlipWand", …)} —
+ * same pattern as {@code ExitInstanceInteraction} (cf. 04-00 SPIKE-RESULT,
+ * Finding 3). The matching {@code "Type": "GravityFlipWand"} reference in
+ * {@code Items/gravityflip_wand.json} wires the click packet to this class.
+ *
+ * <p><b>One class for both click types :</b> {@link #firstRun} receives the
+ * {@link InteractionType}. Primary → {@code setPos1}, Secondary → {@code setPos2}.
+ * Centralising both in one class keeps a single registration entry and a
+ * single CODEC — any other split would duplicate wiring for no gain.
+ *
+ * <p><b>Store injection :</b> the Hytale CODEC instantiates interactions via a
+ * no-arg constructor, so we cannot inject the store through the constructor.
+ * Instead, {@link #bindStore(WandSelectionStore)} installs a {@code volatile}
+ * reference at plugin start. If the store is not bound (mis-wired plugin),
+ * {@link #firstRun} is a safe no-op — fail-silent.
+ *
+ * <p><b>Threat surface :</b>
+ * <ul>
+ *   <li>Chat feedback uses {@link PlayerRef#sendMessage} — directed to the
+ *       clicker only, no broadcast (T-04-01-03).</li>
+ *   <li>{@code TargetBlock} coordinates are stored raw as {@code int[]} — no
+ *       numeric processing in this plan; {@code /gravityflip define} (04-03)
+ *       is responsible for clamping / validating before region creation
+ *       (T-04-01-01).</li>
+ * </ul>
+ */
+public final class GravityFlipWandInteraction extends SimpleInstantInteraction {
+
+    @Nonnull
+    public static final BuilderCodec<GravityFlipWandInteraction> CODEC =
+            ((BuilderCodec.Builder) BuilderCodec
+                    .builder(GravityFlipWandInteraction.class,
+                            GravityFlipWandInteraction::new,
+                            SimpleInstantInteraction.CODEC)
+                    .documentation(
+                            "Gravity Flip wand: Primary click sets pos1, "
+                                    + "Secondary click sets pos2, for the clicker's selection."))
+                    .build();
+
+    /**
+     * Store shared by every instance of this interaction — injected at plugin
+     * start via {@link #bindStore}. Volatile so the writer thread ({@code setup()})
+     * publishes a safe reference to the reader threads (interaction dispatch).
+     */
+    private static volatile WandSelectionStore STORE;
+
+    /** Wire the selection store before any click can be processed. Call once at {@code setup()}. */
+    public static void bindStore(WandSelectionStore store) {
+        STORE = store;
+    }
+
+    /** Required no-arg constructor used by the CODEC factory. */
+    public GravityFlipWandInteraction() {
+    }
+
+    @Override
+    protected void firstRun(@Nonnull InteractionType type,
+                            @Nonnull InteractionContext context,
+                            @Nonnull CooldownHandler cooldownHandler) {
+        WandSelectionStore store = STORE;
+        if (store == null) {
+            // Plugin mis-wired (bindStore never called). Silent no-op — don't crash the click.
+            return;
+        }
+
+        Ref<EntityStore> entityRef = context.getEntity();
+        CommandBuffer<EntityStore> commandBuffer = context.getCommandBuffer();
+        if (entityRef == null || commandBuffer == null) {
+            return;
+        }
+
+        PlayerRef playerRef = commandBuffer.getComponent(entityRef, PlayerRef.getComponentType());
+        if (playerRef == null) {
+            // Clicker is not a player (mob, arrow, …) — ignore.
+            return;
+        }
+
+        BlockPosition bp = context.getTargetBlock();
+        if (bp == null) {
+            playerRef.sendMessage(Message.raw("[gravityflip] No block targeted."));
+            return;
+        }
+
+        UUID uuid = playerRef.getUuid();
+        if (type == InteractionType.Primary) {
+            store.setPos1(uuid, bp.x, bp.y, bp.z);
+            playerRef.sendMessage(Message.raw(
+                    "[gravityflip] pos1 set: (%d, %d, %d)".formatted(bp.x, bp.y, bp.z)));
+        } else if (type == InteractionType.Secondary) {
+            store.setPos2(uuid, bp.x, bp.y, bp.z);
+            playerRef.sendMessage(Message.raw(
+                    "[gravityflip] pos2 set: (%d, %d, %d)".formatted(bp.x, bp.y, bp.z)));
+        }
+        // Any other InteractionType (Ability1, Pick, Equipped, …) is ignored.
+    }
+}