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/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);