public subroutine add_scalar_x_face(f, c, i_plane, nx, ny, nz)
Adds a uniform constant c to a single x-normal plane (pencil index
i_plane) of a DIR_X VERT field, in place.
Launch convention (matches field_set_x_face exactly):
threads = dim3(64, 1, 1)
blocks = dim3((SZ-1)/64 + 1, n_y_blocks*nz, 1)
Threads beyond the real pencil width (SZ in interior y-blocks,
n_mod in the last y-block) are masked out by the i_max guard.
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