create new directories under ./src/sisudoc ocda & outputs in order to
separate the document abstraction library from downstream output
processing (stuff broken till paths & modules fixed)

phase0 step2: move curation modules from meta/ to io_out/curate/

Curation modules moved to src/sisudoc/io_out/curate/, module
declarations renamed sisudoc.io_out.curate.metadoc_curate* from
sisudoc.meta.metadoc_curate* and updated spine.d imports. File contents
are otherwise unchanged.

Completes phase0: meta/ now has zero io_out imports - the abstraction
core's outgoing deps are now only:
  meta/ internals + io_in/ + ext_depends/D-YAML

(assisted by Claude-Code)

phase0: drop vestigial io_out.hub coupling from meta/metadoc.d

phase0 step1: abstraction-library extraction/decoupling: meta/ should
not import io_out/.

Removed unused call to `import sisudoc.io_out.hub;` `mixin outputHub;`
from `template spineAbstraction()`. (the load-bearing UFCS site is
spine.d:92 which has its own `mixin outputHub).

(assisted by Claude-Code)

- odd hilighting issue ... must result from my org config, but "fix"
  makes things easier for me.

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)

- revisit links (fix later)

- preferable, endnote parent object number
  available for use (as here in text output,
  compare "endnotes, add caller ocn" commit)

- spine --text [--output=output path] [markup source]