feat(02-01): add GravityFlipRegion POJO + BuilderCodec with round-trip tests

- GravityFlipRegion holds Name (String) + Box (Hytale AABB) + Enabled (boolean),
  serialised through a BuilderCodec keyed on "Name"/"Box"/"Enabled".
- Validators.nonNull() applied to Name and Box (correct package is
  com.hypixel.hytale.codec.validation.Validators, not codec.builder).
- Server jar 2026.03.26 uses com.hypixel.hytale.math.vector.Vector3d for Box.min/max
  (NOT org.joml.Vector3d, which only appeared in newer builds).
- 3 JUnit 5 round-trip tests cover name/box/enabled, enabled=false, and empty name.
- testImplementation added for the Server artifact so codec encode/decode is
  reachable from unit tests.

build.gradle.kts

@@ -24,6 +24,8 @@ dependencies {
     implementation("com.google.code.gson:gson:2.10.1")
     implementation("org.jetbrains:annotations:24.1.0")
 
+    // Tests need the Hytale Server API (codecs, ExtraInfo, BsonValue) on the classpath.
+    testImplementation("com.hypixel.hytale:Server:$hytaleServerVersion")
     testImplementation("org.junit.jupiter:junit-jupiter:5.10.0")
     testRuntimeOnly("org.junit.platform:junit-platform-launcher")
 }