add children_headings to document abstraction

Add int[] children_headings field to DocObj_MetaInfo_ and
compute it in the post-processing pass of metadoc_from_src.d,
right after last_descendant_ocn. Single O(n) pass builds a
parent_ocn -> child heading OCNs map, then assigns to each
heading object. Useful for tree-structured output.

The .ssp serializer now reads directly from the abstraction
field instead of pre-computing its own map.

  metadoc_object_setter.d: +1 line (field declaration)
  metadoc_from_src.d: +17 lines (computation)
  create_abstraction_txt.d: -10 lines (simplified)

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

0 files changed, 0 insertions, 0 deletions