Enforce 300 ft minimum separation for CL1/CL2/CL3 in cloud-levels.lua

[Copilot]

cloud-levels.lua could still emit CL1/CL2/CL3 bases closer than the required 300 ft after its layer selection and fallback reshuffling. This change adds a final spacing guard so the published cloud layers always satisfy the minimum separation rule.

• What changed

  • Added a final normalization pass over CL1, CL2, and CL3 after the existing layer-selection logic.
  • If two adjacent layers are closer than dz = 300, the pass drops one layer and compacts the remaining outputs.

• Selection rule for conflicts

  • Preserve the lower layer when it is BKN+.
  • Otherwise preserve the upper layer when it is BKN+.
  • If neither layer is BKN+, preserve the lower layer.

• Why this is needed

  • Several existing branches assume replacement layers are already spaced far enough apart.
  • Those assumptions allow invalid close pairs to survive into the final output in some locations.

• Implementation detail

  if bases[j + 1] - bases[j] < dz then
    if IsBknLayer(covs[j]) or not IsBknLayer(covs[j + 1]) then
      DropLayer(bases, tops, covs, j + 1)
    else
      DropLayer(bases, tops, covs, j)
    end
  end