- PendingEntry stores parent_request_id (Call and Subscribe) and started flag
for abort-cascade tree indexing
- register_call/register_subscribe accept optional parent_request_id
- AbortCascade::cascade_abort walks the call tree by parent_request_id and
aborts descendants per AbortPolicy (AbortDependents: all; ContinueRunning:
unstarted only). Returns sorted list of aborted request IDs
- call.aborted for unknown request_id silently discarded (empty result)
- Composed child request_ids stay internal (not sent as call.requested)
- mark_started() tracks dispatch state for ContinueRunning decisions
- 20 unit tests covering AbortDependents/ContinueRunning, depth-3 tree,
unknown root, mixed Call/Subscribe, determinism