feat: Add grid-based player movement manager, new 'stop_n_go' arena, and core game scene setup.

scenes/main.gd

@@ -108,6 +108,19 @@ func _apply_arena_background():
 		"Stop N Go Arena":
 			texture_path = "res://assets/graphics/level_bg/placeholder_stop_n_go.jpg"
 			_instantiate_3d_arena("res://scenes/arena/stop_n_go.scn")
+			
+			# Make normal, non-walkable, and Tekton floors invisible 
+			# by shrinking their scale to 0. This hides them visually while 
+			# avoiding "Parameter 'mesh' is null" or material override errors.
+			var em = $EnhancedGridMap
+			if em and em.mesh_library:
+				var ml = em.mesh_library.duplicate()
+				for id in [0, 4, 6]:
+					# Scale to 0 to hide it without triggering invalid mesh errors
+					ml.set_item_mesh_transform(id, Transform3D().scaled(Vector3.ZERO))
+				
+				em.mesh_library = ml
+				print("[Main] Hide tiles 0, 4, 6 via zero-scale transform.")
 		"Tekton Doors Arena":
 			texture_path = "res://assets/graphics/level_bg/placeholder_tekton_doors.jpg"
 		"Classic", _: