buildLayout now builds the family forest, packs it bottom-up via tidyTree, and
maps each unit's run x back to per-person positions (x from structure, y from
rank). assignRanks, the generations map, and computeViewBox are reused
unchanged. The unknown-id guard now covers PARENT_OF as well as SPOUSE_OF, and
displaced cross-level edges are exposed as crossLinks for distinct rendering.
The ~210-line block packer (and its block/merge helpers) is gone.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>