feat(call): implement abort cascade for nested calls (ADR-016) (task: call/protocol/abort-cascade)

- 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

crates/alknet-call/src/protocol/adapter.rs

@@ -363,6 +363,7 @@ mod tests {
         )
     }
 
+    #[allow(dead_code)]
     fn internal_spec(name: &str) -> OperationSpec {
         OperationSpec::new(
             name,