buildLayout switches to a two-stage assignment:
1. Seed — every node with node.generation != null is locked at that
rank. The fallback heuristic never moves a locked rank, and the
spouse-pulldown never pulls a locked rank.
2. Fallback — for unseeded nodes, rank = max(parent rank) + 1 reading
parents from the same unified rank map, so an unseeded child of a
seeded G 2 parent correctly inherits rank 3. Spouse-pulldown ties
unseeded spouses to their deeper partner exactly as before.
3. Normalise — if any rank is negative (future G −1 ancestor), shift
the whole map so min(rank) == 0. No-op for today's data.
Fixes the Herbert Cram pattern from #361's review: two parented
spouses with imported G 3 now render on the same y row. Existing
StammbaumTree tests still pass byte-for-byte because every test node
has node.generation undefined, so the heuristic runs unchanged.
Refs #689
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
