refactor: add struct constraint to Relationship

Add `where TSelf : struct where TTarget : struct` constraints to
the `Relationship` struct to ensure type safety and consistency with
the ECS architecture. Also remove unused `IterateThree` method in
`QueryExecutor`.

src/OECS/QueryExecutor.cs

@@ -179,48 +179,6 @@ internal static class QueryExecutor
         }
     }
 
-    private static void IterateThree<TDriver, TA, TB>(
-        SparseSet<TDriver> driveSet,
-        SparseSet<TA> setA,
-        SparseSet<TB> setB,
-        ComponentStore store,
-        IReadOnlySet<Type> withoutTypes,
-        ForEachAction<TA, TDriver, TB> actionWhenDriverIsT2,
-        ForEachAction<TDriver, TA, TB> actionWhenDriverIsT1,
-        ForEachAction<TA, TB, TDriver> actionWhenDriverIsT3)
-        where TDriver : struct where TA : struct where TB : struct
-    {
-        var dense = driveSet.Dense;
-        var denseEntities = driveSet.DenseEntities;
-        var count = driveSet.Count;
-
-        for (int i = 0; i < count; i++)
-        {
-            var entity = denseEntities[i];
-            if (entity.Id == SingletonId) continue;
-            if (!setA.Contains(entity)) continue;
-            if (!setB.Contains(entity)) continue;
-            if (!PassesWithoutFilter(store, entity, withoutTypes))
-                continue;
-
-            if (actionWhenDriverIsT2 != null!)
-            {
-                // Driver is T2, so order is: T1=setA, T2=driveSet, T3=setB
-                actionWhenDriverIsT2(entity, ref setA.Get(entity), ref dense[i], ref setB.Get(entity));
-            }
-            else if (actionWhenDriverIsT3 != null!)
-            {
-                // Driver is T3, so order is: T1=setA, T2=setB, T3=driveSet
-                actionWhenDriverIsT3(entity, ref setA.Get(entity), ref setB.Get(entity), ref dense[i]);
-            }
-            else
-            {
-                // Driver is T1, so order is: T1=driveSet, T2=setA, T3=setB
-                actionWhenDriverIsT1(entity, ref dense[i], ref setA.Get(entity), ref setB.Get(entity));
-            }
-        }
-    }
-
     public static void ForEach<T1, T2, T3, T4>(
         ComponentStore store,
         QueryDescriptor query,

src/OECS/Relationship.cs

@@ -25,6 +25,7 @@ namespace OECS;
 /// </summary>
 [MessagePackObject]
 public struct Relationship<TSelf, TTarget> : IRelationship
+    where TSelf : struct where TTarget : struct
 {
     /// <summary>
     /// The entity that owns this relationship component.

tests/OECS.Tests/RelationshipTests.cs

@@ -9,6 +9,7 @@ public struct ChildOf { }
 public struct ParentOf { }
 public struct OwnedBy { }
 public struct MemberOf { }
+public struct Thing { }
 
 // ── Tests ────────────────────────────────────────────────────────────
 
@@ -153,20 +154,20 @@ public class RelationshipTests
         var owner = world.CreateEntity();
         var group = world.CreateEntity();
 
-        world.AddComponent(entity, new Relationship<OwnedBy, object>
+        world.AddComponent(entity, new Relationship<OwnedBy, Thing>
         {
             Source = entity, Target = owner
         });
-        world.AddComponent(entity, new Relationship<MemberOf, object>
+        world.AddComponent(entity, new Relationship<MemberOf, Thing>
         {
             Source = entity, Target = group
         });
 
-        world.GetSources<Relationship<OwnedBy, object>>(owner).Should().BeEquivalentTo([entity]);
+        world.GetSources<Relationship<OwnedBy, Thing>>(owner).Should().BeEquivalentTo([entity]);
-        world.GetSources<Relationship<MemberOf, object>>(group).Should().BeEquivalentTo([entity]);
+        world.GetSources<Relationship<MemberOf, Thing>>(group).Should().BeEquivalentTo([entity]);
 
         // Cross-check: OwnedBy sources should not appear in MemberOf lookups.
-        world.GetSources<Relationship<MemberOf, object>>(owner).Should().BeEmpty();
+        world.GetSources<Relationship<MemberOf, Thing>>(owner).Should().BeEmpty();
     }
 
     [Fact]