NEON-29: explicitly set HorizontalStepEnabled:false in appsettings

Makes the no-horizontal-limit intent explicit in config rather than
relying on the C# class default; prevents accidental re-enabling if
the default ever changes.

server/NeonSprawl.Server/appsettings.json

@@ -14,6 +14,7 @@
       "Z": -5
     },
     "MovementValidation": {
+      "HorizontalStepEnabled": false,
       "MaxHorizontalStep": 18,
       "MaxVerticalStep": 1.25,
       "DistrictBoundsEnabled": false