This attribute exposes mesh vertex normals as a `vertex_normal` attribute for use with nodes. Since the normal vector stored in vertices is only a cache of data computable from the surrounding faces, the attribute is read-only. A proper error message for attempting to write this attribute is part of T85749. A write-only normal attribute will likely come later, most likely called `corner_normal`. The normals are recomputed before reading if they are marked dirty. This involves const write-access to the mesh, protected by the mutex stored in `Mesh_Runtime`. This is essential for correct behavior after nodes like "Edge Split" or nodes that adjust the position attribute. Ref T84297, T85880, T86206 Differential Revision: https://developer.blender.org/D10541