fix: enable collision pairs, correct physics shape sizes, lower floor

- ObjectLayerPairFilterTable blocks all collisions by default in C# binding;
  explicitly EnableCollision for all layer pairs (NonMoving/Moving)
- BoxShape takes half-extents, not full size: DynamicBox(scale * 0.5f)
- Floor visual thickness reduced (scale Y 1→0.5), top at y=0 matching collider
- Floor lowered to y=-0.5
- Physics works: cubes and spheres fall and land on floor, no clipping

src/CortexEngine.App/Program.cs

@@ -322,7 +322,7 @@ class Program
                 .Set(new Transform(pos, Quaternion.Identity, new Vector3(scale)))
                 .Set(mesh)
                 .Set(new Material(color, roughness: rough, metallic: metal))
-                .Set(RigidBody.DynamicBox(new Vector3(scale), mass: scale * 2f));
+                .Set(RigidBody.DynamicBox(new Vector3(scale * 0.5f), mass: scale * 2f));
         }
 
         var spheres = new (string name, Vector3 pos, Vector3 color, float scale, float rough, float metal)[]
@@ -352,7 +352,7 @@ class Program
             .Set(new Material(new Vector3(0.8f, 0.8f, 0.85f), roughness: 0.4f, metallic: 0.0f, texturePath: "Content/checker.png"));
 
         world.Entity("Floor")
-            .Set(new Transform(new Vector3(0, -0.02f, 0), Quaternion.Identity, new Vector3(20, 1, 20)))
+            .Set(new Transform(new Vector3(0, -0.5f, 0), Quaternion.Identity, new Vector3(20, 0.5f, 20)))
             .Set(mesh)
             .Set(new Material(new Vector3(0.45f, 0.45f, 0.5f), roughness: 0.8f, metallic: 0.0f))
             .Set(RigidBody.StaticPlane(20f));

src/Engine.Physics/PhysicsWorld.cs

@@ -46,6 +46,11 @@ public sealed class PhysicsWorld : IDisposable
         _broadPhaseLayerInterface.MapObjectToBroadPhaseLayer(Layers.Moving, new BroadPhaseLayer(1));
 
         _objectLayerPairFilter = new ObjectLayerPairFilterTable(2);
+        // Explicitly enable all collision pairs
+        _objectLayerPairFilter.EnableCollision(Layers.NonMoving, Layers.NonMoving);
+        _objectLayerPairFilter.EnableCollision(Layers.NonMoving, Layers.Moving);
+        _objectLayerPairFilter.EnableCollision(Layers.Moving, Layers.NonMoving);
+        _objectLayerPairFilter.EnableCollision(Layers.Moving, Layers.Moving);
 
         _objectVsBroadPhaseLayerFilter = new ObjectVsBroadPhaseLayerFilterTable(
             _broadPhaseLayerInterface, 2, _objectLayerPairFilter, 2);