Implements TicketGraph<'a> with build(), roots(), subtree(),
to_json_value(), and to_subtree_json_value(). Tracks both forward
(dependencies) and reverse (dependents) edges. Cycle-safe DFS via
visited set. No new crate dependencies. 14 unit tests covering empty
graph, chains, branching, cycles, dangling refs, and JSON output.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Elijah Voigt