The is_descendant_of and apply_set_parent cycle detection both used
hand-rolled 32-hop bounded ancestor walks. Correct in practice (no
real hive exceeds 32 levels) but carried an arbitrary ceiling and were
harder to reason about than proven graph primitives.
Changes:
- Add build_graph(): converts BTreeMap<name, parent|null> → DiGraph
with parent→child edges + BTreeMap<name, NodeIndex> index
- Add is_descendant_of_in(): pure (no disk I/O), uses
petgraph::algo::has_path_connecting from ancestor to candidate
- Rewrite is_descendant_of(): delegates to is_descendant_of_in(&read())
- Rewrite apply_set_parent() cycle detection: build_graph() + speculative
edge + is_cyclic_directed(); no depth limit
- Add tests for is_descendant_of_in (self, direct child, grandchild,
parent-is-not-child, sibling, unknown)
petgraph was already a workspace dep (used elsewhere). On-disk format
unchanged (flat JSON map). Public API surface unchanged.
