feat(02-01): wire GravityFlipConfig to regions.json via named withConfig

- GravityFlipConfig wraps List<GravityFlipRegion> via ArrayCodec under the
  KeyedCodec("Regions", ...) entry. Decoded list is a mutable ArrayList
  (NOT List.of(arr)) so Phase 4 commands can mutate at runtime.
- GravityFlipPlugin.configHolder uses the named overload
  withConfig("regions", GravityFlipConfig.CODEC) — produces
  <dataDirectory>/regions.json. The 1-arg overload would hardcode config.json.
- configHolder() javadoc documents the save contract (no auto-save on
  shutdown; mutators must call configHolder.save() explicitly) and the shared
  mutable reference caveat that 02-02's RegionRegistry will resolve.
- 4 round-trip tests cover: empty default, two-region order preservation,
  empty list, and list mutability (regression guard against List.of).

src/test/java/com/mythlane/gravityflip/config/GravityFlipConfigCodecTest.java

@@ -0,0 +1,92 @@
+package com.mythlane.gravityflip.config;
+
+import com.hypixel.hytale.codec.ExtraInfo;
+import com.hypixel.hytale.math.shape.Box;
+import com.hypixel.hytale.math.vector.Vector3d;
+import com.mythlane.gravityflip.region.GravityFlipRegion;
+import org.bson.BsonValue;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Round-trip tests for {@link GravityFlipConfig#CODEC}. Critical guarantees:
+ * <ul>
+ *   <li>Regions list is always non-null (empty by default).</li>
+ *   <li>List elements survive encode -> decode in order.</li>
+ *   <li>Decoded list is MUTABLE — Phase 4 command handlers depend on this.
+ *       Guard against an accidental {@code List.of(arr)} regression.</li>
+ * </ul>
+ */
+class GravityFlipConfigCodecTest {
+
+    @Test
+    void newConfigHasNonNullEmptyRegions() {
+        GravityFlipConfig cfg = new GravityFlipConfig();
+        assertNotNull(cfg.getRegions());
+        assertEquals(0, cfg.getRegions().size());
+    }
+
+    @Test
+    void roundTripPreservesTwoRegionsInOrder() {
+        GravityFlipConfig src = new GravityFlipConfig();
+        src.getRegions().add(region("alpha", 0, 0, 0, 1, 1, 1));
+        src.getRegions().add(region("beta", 5, 5, 5, 9, 9, 9));
+
+        GravityFlipConfig decoded = roundTrip(src);
+
+        assertEquals(2, decoded.getRegions().size());
+        assertEquals("alpha", decoded.getRegions().get(0).getName());
+        assertEquals("beta", decoded.getRegions().get(1).getName());
+    }
+
+    @Test
+    void roundTripOfEmptyListYieldsNonNullEmptyList() {
+        GravityFlipConfig src = new GravityFlipConfig();
+        // src.regions is the default empty ArrayList.
+        GravityFlipConfig decoded = roundTrip(src);
+
+        assertNotNull(decoded.getRegions(), "decoded regions list must never be null");
+        assertEquals(0, decoded.getRegions().size());
+    }
+
+    @Test
+    void decodedListIsMutable() {
+        GravityFlipConfig src = new GravityFlipConfig();
+        src.getRegions().add(region("seed", 0, 0, 0, 1, 1, 1));
+
+        GravityFlipConfig decoded = roundTrip(src);
+
+        // CRITICAL: must not throw UnsupportedOperationException.
+        // Phase 4 commands (define / delete / toggle) all mutate this list.
+        assertDoesNotThrow(() -> decoded.getRegions().add(region("added", 2, 2, 2, 3, 3, 3)));
+        assertDoesNotThrow(() -> decoded.getRegions().remove(0));
+        assertTrue(decoded.getRegions() instanceof ArrayList,
+                "decoded regions list should be a mutable ArrayList, not List.of(...)");
+    }
+
+    private static GravityFlipRegion region(String name,
+                                            double minX, double minY, double minZ,
+                                            double maxX, double maxY, double maxZ) {
+        return new GravityFlipRegion(
+                name,
+                new Box(new Vector3d(minX, minY, minZ), new Vector3d(maxX, maxY, maxZ)),
+                true);
+    }
+
+    private static GravityFlipConfig roundTrip(GravityFlipConfig src) {
+        ExtraInfo info = new ExtraInfo();
+        BsonValue encoded = GravityFlipConfig.CODEC.encode(src, info);
+        return GravityFlipConfig.CODEC.decode(encoded, info);
+    }
+
+    // Suppress unused-import warning if List is not directly referenced in any final assertion.
+    @SuppressWarnings("unused")
+    private static List<GravityFlipRegion> typeAnchor() { return null; }
+}