vmix_seed_backgrounds Subroutine

   pure subroutine vmix_seed_backgrounds(this, skip_kv)
      !! Seed `kv_bg`/`kt_bg`/`ks_bg` and the `kv`/`kt`/`ks`/`kd_bg` arrays
      !! from the current `pp81_nu_bg`/`pp81_kappa_bg` fields, then zero
      !! the closed-BC boundary interfaces on `kv`/`ks`.  Extracted out of
      !! `ocean_vmix_init` (§2E structural invariant: `kv_bg == pp81_nu_bg`,
      !! `kt_bg == ks_bg == pp81_kappa_bg`, so `vmix_assemble`'s background
      !! floor is a no-op for the shipped PP81 closure) so BOTH the initial
      !! seed and the `&ocean_vmix_nml pp81_*` config-copy re-derive
      !! consistently — a bare field copy without this call would leave
      !! the assembly floor clamped against the OLD (type-default)
      !! background even after a user sets a new one.  Requires
      !! `kv`/`kt`/`ks`/`kd_bg` already allocated (true after `init`; the
      !! config-copy call runs strictly after `init_from_config`).
      class(ocean_vmix_t), intent(inout) :: this
      logical, intent(in), optional :: skip_kv
         !! PR-2 (bt-rem-from-av-rem, fixed per review): default `.false.`
         !! — the FULL seed always runs (scalars + `kv`/`kt`/`ks`/`kd_bg`
         !! arrays + the `kv`/`ks` boundary zero), exactly the historical
         !! behaviour.  The config-copy call site
         !! (`configure_ocean_lateral`, AFTER `engine_setup`'s restart
         !! read) passes `skip_kv = state%vmix%kv_from_restart` — `.true.`
         !! ONLY when THIS read actually found `vmix_kv` in the checkpoint
         !! (an older checkpoint without the field, or a cold start, both
         !! leave `kv_from_restart = .false.`, so the array still reseeds
         !! normally and the run is never left with an uninitialised
         !! `kv`).  When skipped, `kv` is left EXACTLY as the restart read
         !! wrote it — no reseed, no boundary re-zero — because a
         !! checkpointed `kv` is a CARRIED field (`visc_rem_precompute`
         !! reads the PREVIOUS stage's `kv` before this stage recomputes
         !! it) and re-zeroing its boundary rows is not provably
         !! idempotent: nothing in this tree asserts every `kv`-writing
         !! closure (PP81/KPP/EPBL/kappa-shear/tidal-mixing/convective
         !! adjustment, `vmix_assemble`) keeps `kv(:,:,1)` /
         !! `kv(:,:,nz+1)` at exactly 0 throughout a run, so re-asserting
         !! it here could diverge a restored run from the continued one
         !! it must match bitwise. Restoring the checkpoint verbatim is
         !! the only choice that is unconditionally correct.
         !!
         !! The FIRST bug report on this knob (then named `reseed_arrays`)
         !! was wrong in a different way: it skipped `kt`/`ks`/`kd_bg` and
         !! the boundary zero TOO, so a warm restart lost the background
         !! diffusivity entirely (`kd_bg` is set ONLY here) whenever a
         !! user ran with `bt_rem_from_visc_rem` and a Bryan-Lewis/Henyey
         !! background or a closed-BC config.  `kt`/`ks`/`kd_bg` are
         !! never restart-registry state (no cross-stage read lags them,
         !! unlike `kv`), so they — and the scalar trackers — always
         !! reseed from the nml-configured `pp81_*`, cold or warm,
         !! unconditionally.
      logical :: do_skip_kv
      integer :: nz1

      do_skip_kv = .false.
      if (present(skip_kv)) do_skip_kv = skip_kv

      this%kv_bg = this%pp81_nu_bg
      this%kt_bg = this%pp81_kappa_bg
      this%ks_bg = this%pp81_kappa_bg

      this%kt = this%pp81_kappa_bg
      this%ks = this%pp81_kappa_bg
      this%kd_bg = this%pp81_kappa_bg

      ! Zero the closed-BC boundary interfaces on ks unconditionally (ks
      ! is never restart state).
      nz1 = size(this%ks, 3)
      this%ks(:, :, 1) = 0.0_wp
      this%ks(:, :, nz1) = 0.0_wp

      if (do_skip_kv) return

      this%kv = this%pp81_nu_bg
      this%kv(:, :, 1) = 0.0_wp
      this%kv(:, :, nz1) = 0.0_wp
   end subroutine vmix_seed_backgrounds