fill_v_centre_impl Subroutine

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