parse_diag_spec Function

   function parse_diag_spec(spec) result(specs)
      !! Parse the `&ocean_diag_nml diags` selection string into structured
      !! entries.  Entries are whitespace/comma-separated; within an entry,
      !! colon-separated attributes are self-identifying (order-free):
      !!
      !!   * `off`                        -> skip the diagnostic
      !!   * `instant`/`mean`/`max`/`min` -> time-operator override
      !!   * `<int><unit>` (s/m/h/d)      -> cadence override
      !!
      !! The first colon field is always the diagnostic name (case-sensitive,
      !! matched against registered / catalog names).  Attribute matching is
      !! case-insensitive.  An empty/blank string yields a zero-length array
      !! (the bit-identical default).  An unrecognised attribute fails loud.
      character(len=*), intent(in) :: spec
      type(diag_spec_t), allocatable :: specs(:)
      character(len=len(spec)) :: buf
      integer :: i, j, n, ntok, k

      ! Normalise commas to spaces so both separators work.
      buf = spec
      do i = 1, len(buf)
         if (buf(i:i) == ",") buf(i:i) = " "
      end do
      n = len_trim(buf)

      ! Pass 1: count whitespace-delimited tokens.
      ntok = 0
      i = 1
      do while (i <= n)
         if (buf(i:i) == " ") then
            i = i + 1
            cycle
         end if
         ntok = ntok + 1
         do while (i <= n)
            if (buf(i:i) == " ") exit
            i = i + 1
         end do
      end do

      allocate (specs(ntok))
      if (ntok == 0) return

      ! Pass 2: parse each token into name + attributes.
      k = 0
      i = 1
      do while (i <= n)
         if (buf(i:i) == " ") then
            i = i + 1
            cycle
         end if
         j = i
         do while (j <= n)
            if (buf(j:j) == " ") exit
            j = j + 1
         end do
         k = k + 1
         call parse_one_spec_token(buf(i:j - 1), specs(k))
         i = j + 1
      end do
   end function parse_diag_spec