refactor: replace SpaceLayerKinds with strongly-typed SpaceLayerKind enum

Replaces string-based `SpaceLayerKinds` constants with a strongly-typed `SpaceLayerKind` enum. Updates backend services, runtime models, and projection logic to use the new enum. Adds `ToContractValue` method for compatibility with existing contracts.

apps/backend/Shared/Runtime/SimulationKinds.cs

@@ -86,12 +86,12 @@ public enum ShipState
     Fleeing,
 }
 
-public static class SpaceLayerKinds
+public enum SpaceLayerKind
 {
-    public const string UniverseSpace = "universe-space";
-    public const string GalaxySpace = "galaxy-space";
-    public const string SystemSpace = "system-space";
-    public const string LocalSpace = "local-space";
+    UniverseSpace,
+    GalaxySpace,
+    SystemSpace,
+    LocalSpace,
 }
 
 public static class MovementRegimeKinds
@@ -274,4 +274,13 @@ public static class SimulationEnumMappings
         ShipState.Fleeing => "fleeing",
         _ => throw new ArgumentOutOfRangeException(nameof(state), state, null),
     };
+
+    public static string ToContractValue(this SpaceLayerKind kind) => kind switch
+    {
+        SpaceLayerKind.UniverseSpace => "universe-space",
+        SpaceLayerKind.GalaxySpace => "galaxy-space",
+        SpaceLayerKind.SystemSpace => "system-space",
+        SpaceLayerKind.LocalSpace => "local-space",
+        _ => throw new ArgumentOutOfRangeException(nameof(kind), kind, null),
+    };
 }