feat(04-01): implement WandSelectionStore pure-data per-UUID selection store

- ConcurrentHashMap<UUID, Selection>, atomic compute() for set/get
- Selection immutable holder with isComplete()
- No Hytale imports; testable standalone

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

@@ -0,0 +1,82 @@
+package com.mythlane.gravityflip.wand;
+
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * Thread-safe per-player wand selection store. Tracks the two corner positions
+ * ({@code pos1} = Primary click, {@code pos2} = Secondary click) of each
+ * builder's current selection, keyed by player {@link UUID}.
+ *
+ * <p><b>Pure-data :</b> no Hytale runtime dependency — same philosophy as
+ * {@code FallDamageGuard}. Testable with JUnit alone, reusable by future
+ * {@code /gravityflip define} command (Phase 04-02+) without runtime coupling.
+ *
+ * <p><b>Thread-safety :</b> backed by a {@link ConcurrentHashMap} whose
+ * {@code compute(...)} mutations are atomic. Safe for concurrent
+ * {@link #setPos1}/{@link #setPos2} calls on the same UUID (STRIDE T-04-01-04).
+ *
+ * <p><b>Lifecycle :</b> in-memory only — selections are discarded on plugin
+ * shutdown. Conscious design : a builder will not quit mid-{@code define}.
+ */
+public final class WandSelectionStore {
+
+    /** Immutable holder for a (possibly partial) selection. */
+    public static final class Selection {
+        /** {@code {x,y,z}} of the Primary click, or {@code null} if unset. */
+        public final int[] pos1;
+        /** {@code {x,y,z}} of the Secondary click, or {@code null} if unset. */
+        public final int[] pos2;
+
+        Selection(int[] pos1, int[] pos2) {
+            this.pos1 = pos1;
+            this.pos2 = pos2;
+        }
+
+        /** True iff both corners have been picked. */
+        public boolean isComplete() {
+            return pos1 != null && pos2 != null;
+        }
+    }
+
+    private static final Selection EMPTY = new Selection(null, null);
+
+    private final ConcurrentHashMap<UUID, Selection> byUuid = new ConcurrentHashMap<>();
+
+    /** Record the Primary-click corner for {@code uuid}. Keeps any existing pos2. */
+    public void setPos1(UUID uuid, int x, int y, int z) {
+        if (uuid == null) return;
+        byUuid.compute(uuid, (k, prev) -> new Selection(
+                new int[]{x, y, z},
+                prev != null ? prev.pos2 : null));
+    }
+
+    /** Record the Secondary-click corner for {@code uuid}. Keeps any existing pos1. */
+    public void setPos2(UUID uuid, int x, int y, int z) {
+        if (uuid == null) return;
+        byUuid.compute(uuid, (k, prev) -> new Selection(
+                prev != null ? prev.pos1 : null,
+                new int[]{x, y, z}));
+    }
+
+    /**
+     * Return the current selection for {@code uuid}. Never returns {@code null} :
+     * an unknown UUID yields a {@link Selection} with both corners {@code null}.
+     */
+    public Selection get(UUID uuid) {
+        if (uuid == null) return EMPTY;
+        Selection s = byUuid.get(uuid);
+        return s != null ? s : EMPTY;
+    }
+
+    /** Forget the selection for {@code uuid} (e.g. after {@code /gravityflip define}). */
+    public void clear(UUID uuid) {
+        if (uuid == null) return;
+        byUuid.remove(uuid);
+    }
+
+    /** Diagnostic : number of players with an in-flight selection. */
+    public int size() {
+        return byUuid.size();
+    }
+}