Complete region coordinate boundary coverage

src/test/java/com/livingworld/regions/RegionCoordinateTest.java

@@ -148,6 +148,16 @@ class RegionCoordinateTest {
             assertEquals(-2, rc.x());
             assertEquals(0, rc.z());
         }
+
+        @Test
+        void negativeZBoundaryUsesFloorDivision() {
+            assertEquals(
+                    new RegionCoordinate("overworld", 0, -1),
+                    RegionCoordinate.fromBlock("overworld", 0, -128, 8));
+            assertEquals(
+                    new RegionCoordinate("overworld", 0, -2),
+                    RegionCoordinate.fromBlock("overworld", 0, -129, 8));
+        }
     }
 
     // ------------------------------------------------------------------
@@ -375,4 +385,4 @@ class RegionCoordinateTest {
             assertDoesNotThrow(() -> rc.toString());
         }
     }
-}
+}