refactor: Add BitMask256 utility for 256-bit bitmask operations (#2300)

### Summary

- Create BitMask256 struct with scalar operations (benchmarks showed AVX2 vectorization provides no benefit for this size)
- Refactor Map.cs to use BitMask256 for full Z range support (-128 to 127)
- Remove SectorSpawnCache struct, use BitMask256 directly in manager
- Update tests to use BitMask256 directly
- Remove unused test assertions for old 64-bit behavior

Projects/Server.Tests/Tests/Maps/CanSpawnMobileTileDataTests.cs

@@ -23,8 +23,6 @@ public class CanSpawnMobileTileDataTests
         Skip.If(!ServerFixture.TileDataLoaded, "TileData not loaded - client files required");
     }
 
-    #region Multi-Based Tests
-
     [SkippableFact]
     public void CanSpawnMobile_FindsSurfaceOnMulti()
     {
@@ -241,10 +239,6 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    #endregion
-
-    #region Real Map Data Tests
-
     [SkippableFact]
     public void CanSpawnMobile_MalasBuilding_FindsGroundFloor()
     {
@@ -290,10 +284,6 @@ public class CanSpawnMobileTileDataTests
         Assert.True(spawnZ <= -40, $"Expected lowest floor around -50, got {spawnZ}");
     }
 
-    #endregion
-
-    #region Helper Methods and Classes
-
     private TestMulti CreateFloorMulti(Map map, Point3D location, int floorZ)
     {
         var multi = new TestMulti(CreateFloorComponents(floorZ));
@@ -367,6 +357,4 @@ public class CanSpawnMobileTileDataTests
 
         public override MultiComponentList Components => _components;
     }
-
-    #endregion
 }