refactor: remove Source property from IRelationship

Remove the redundant `Source` property from `IRelationship` and its
implementations. Since the relationship component is inherently
stored on the source entity, the `Source` field was unnecessary and
required manual synchronization during deserialization.

This change simplifies the relationship model, reduces memory usage,
and removes the need for reflection-based fixups in the serializer.

OECS/IRelationship.cs

@@ -2,19 +2,14 @@ namespace OECS;
 
 /// <summary>
 /// Marker interface for components that represent a directed relationship
-/// between two entities. The component is stored on the <see cref="Source"/>
-/// entity and points to the <see cref="Target"/> entity.
+/// between two entities. The component is stored on the source entity
+/// (the entity it's added to) and points to the <see cref="Target"/> entity.
 ///
 /// The <see cref="World"/> automatically maintains a reverse index so that
 /// all sources pointing to a given target can be looked up efficiently.
 /// </summary>
 public interface IRelationship
 {
-    /// <summary>
-    /// The entity that owns this relationship component.
-    /// </summary>
-    Entity Source { get; }
-
     /// <summary>
     /// The entity that this relationship points to.
     /// </summary>

OECS/Relationship.cs

@@ -14,7 +14,6 @@ namespace OECS;
 /// // Define a "ChildOf" relationship between a child and a parent entity.
 /// world.AddComponent(child, new Relationship<ChildOf, Parent>
 /// {
-///     Source = child,
 ///     Target = parent
 /// });
 ///
@@ -27,15 +26,9 @@ namespace OECS;
 public struct Relationship<TSelf, TTarget> : IRelationship
     where TSelf : struct where TTarget : struct
 {
-    /// <summary>
-    /// The entity that owns this relationship component.
-    /// </summary>
-    [Key(0)]
-    public Entity Source { get; set; }
-
     /// <summary>
     /// The entity that this relationship points to.
     /// </summary>
-    [Key(1)]
+    [Key(0)]
     public Entity Target { get; set; }
 }

OECS/World.cs

@@ -203,19 +203,8 @@ public class World : IDisposable
         ThrowIfNotAlive(entity);
 
         // If replacing an existing relationship, remove the old index entry first.
-        if (component is IRelationship newRel)
+        if (component is IRelationship)
         {
-            // Guard against mismatched Source: the relationship must be stored
-            // on the entity it claims as its Source, otherwise the reverse
-            // index becomes corrupted.
-            if (newRel.Source != entity)
-            {
-                throw new InvalidOperationException(
-                    $"Relationship of type {typeof(T).Name} has Source={newRel.Source} " +
-                    $"but is being added to entity {entity}. " +
-                    $"The Source must match the entity the component is added to.");
-            }
-
             if (_components.TryGet<T>(entity, out var old))
             {
                 var oldRel = (IRelationship)(object)old;

OECS/WorldSerializer.cs

@@ -1,4 +1,3 @@
-using System.Reflection;
 using MessagePack;
 
 namespace OECS;
@@ -94,32 +93,10 @@ public static class WorldSerializer
                         $"Ensure the component type is used with the World " +
                         $"API so the source generator can discover it.");
 
-                // Deserialize and fix up IRelationship.Source before adding.
+                // Deserialize and add via the typed internal method.
                 var component = desc.Deserialize(ce.Data);
-                if (component is IRelationship rel)
-                    FixupRelationshipSource(rel, entity);
-
-                // Add via the typed internal method — no reflection for the add itself.
                 world.AddComponentBoxed(entity, component, desc.Type);
             }
         }
     }
-
-    /// <summary>
-    /// Sets the Source field on a deserialized IRelationship to match
-    /// the entity it's being restored to. Uses a small cache of PropertyInfo
-    /// to avoid repeated reflection lookups.
-    /// </summary>
-    private static void FixupRelationshipSource(IRelationship rel, Entity entity)
-    {
-        var type = rel.GetType();
-        if (!_sourcePropCache.TryGetValue(type, out var prop))
-        {
-            prop = type.GetProperty("Source");
-            _sourcePropCache[type] = prop;
-        }
-        prop?.SetValue(rel, entity);
-    }
-
-    private static readonly Dictionary<Type, PropertyInfo?> _sourcePropCache = new();
 }