fill_u_centre_impl Subroutine

   pure subroutine fill_u_centre_impl(u_face, buf)
      ! assumed-shape-ok: diag fill — fires once per output frame (cadence-bounded);
      ! face-sized u_face(:,:,:) has nx+1 first dim, incompatible with cell-sized buf.
      real(wp), intent(in)    :: u_face(:, :, :)
      real(wp), intent(inout) :: buf(:, :, :)  ! assumed-shape-ok: diag fill — cadence-bounded
      integer :: i, j, k, nx, ny, nz
      nx = min(size(buf, 1), size(u_face, 1) - 1)
      ny = min(size(buf, 2), size(u_face, 2))
      nz = min(size(buf, 3), size(u_face, 3))
      do concurrent(k=1:nz, j=1:ny, i=1:nx)
         buf(i, j, k) = 0.5_wp*(u_face(i, j, k) + u_face(i + 1, j, k))
      end do
   end subroutine fill_u_centre_impl