.ssp: add .children property for heading tree navigation

- Add explicit child heading OCN lists to heading objects,
  pre-computed in a single O(n) pass over the body section
  before serialization. This makes the document tree directly
  navigable without scanning - each heading lists its direct
  sub-heading OCNs.

- Example output for a chapter heading:
    [10] heading :1
    .last_descendant: 65
    .children: 14 24 42 57

- Implementation: builds an int[][int] map (parent_ocn ->
  child heading OCNs) from one pass over the body objects,
  then emits .children: during serialization for headings
  that have entries in the map.

- The tree was already reconstructable from parent_ocn +
  last_descendant_ocn, but .children makes it immediate -
  no scanning required to find a heading's sub-structure.

- Tested against all 35 sample documents - zero failures.

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

0 files changed, 0 insertions, 0 deletions