.ssp document abstraction as PEG parsable text

  --show-abstraction flag to write .ssp document abstraction files

- Add a new output mode that serializes the in-memory document
  abstraction (produced by spineAbstraction) to a human-readable,
  line-oriented text format (.ssp). This captures the full object
  model after parsing and abstraction but before output generation.

- The .ssp format uses unambiguous line prefixes:
    @section { }  - section boundaries (head/toc/body/endnotes/...)
    [N] type      - object declaration with OCN
    .name: value  - object properties (only non-defaults)
    | content     - text content lines
    % comment     - comments

- New files:
    src/sisudoc/io_out/create_abstraction_txt.d
      Serializer module following the same template pattern as
      metadoc_show_summary.d. Walks doc.abstraction() section by
      section, writing metadata preamble (@meta, @make, @doc_has)
      then each object with its properties and text content.
      Output goes to {output_path}/{lang}/abstraction/{doc}.ssp

- Changes to spine.d:
    - Add "show-abstraction" to opts initialization, getopt, and
      OptActions struct
    - Add show_abstraction to abstraction(), require_processing_files(),
      and meta_processing_general() so the flag triggers full document
      processing
    - Insert call at both spineAbstraction sites (parallel and serial
      branches), gated by show_abstraction flag, following the same
      pattern as show_config/show_summary/show_make

- Tested against all 35 sample documents (including multilingual
  live-manual in 9 languages) - zero failures. Works standalone
  (--show-abstraction) or combined with other output flags
  (--show-abstraction --html --text). No effect on existing code
  paths when the flag is not used.

  Co-Authored-By: Anthropic Claude Opus 4.6 (1M context)

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