feat: Unity-like object manipulation + physics fixes

- ObjectManipulator: left-click to pick, drag on camera-orthogonal plane
- Screen-space movement (like Unity), not ground-plane
- Skip dragged entity in SyncTransforms so physics doesn't fight drag
- Zero velocity on grab to prevent momentum accumulation
- ImGui selection sync with manipulator
- Floor lowered to y=-0.5, visual thickness corrected
- Physics shape sizes fixed (BoxShape uses half-extents)
- Collision pairs explicitly enabled in ObjectLayerPairFilterTable

src/Engine.Physics/PhysicsWorld.cs

@@ -121,7 +121,7 @@ public sealed class PhysicsWorld : IDisposable
         _physicsSystem.Update(deltaTime, collisionSteps, _jobSystem);
     }
 
-    public void SyncTransforms(World world)
+    public void SyncTransforms(World world, Entity? skipEntity = null)
     {
         // Collect updates first to avoid calling entity.Set() during dictionary iteration
         var updates = new List<(Entity entity, Vector3 pos, Quaternion rot)>();
@@ -130,6 +130,9 @@ public sealed class PhysicsWorld : IDisposable
             if ((ulong)entity.Id == 0)
                 continue;
 
+            if (skipEntity.HasValue && skipEntity.Value.Id == entity.Id)
+                continue;
+
             var pos = _bodyInterface.GetPosition(bodyId);
             var rot = _bodyInterface.GetRotation(bodyId);
             updates.Add((entity, pos, rot));
@@ -154,6 +157,8 @@ public sealed class PhysicsWorld : IDisposable
             return;
 
         _bodyInterface.SetPositionAndRotation(bodyId, transform.Position, transform.Rotation, Activation.Activate);
+        // Zero out velocity so the object doesn't accumulate momentum while being dragged
+        _bodyInterface.SetLinearAndAngularVelocity(bodyId, Vector3.Zero, Vector3.Zero);
     }
 
     private static Shape CreateShape(RigidBody rb)