Sub-orchestration losers in select2/select races should be cancelled

[affandar]

Problem

When a sub-orchestration loses a select2() or select() race, it is not cancelled and continues running as an orphan. This is inconsistent with the expected behavior and leads to resource leaks.

Current Behavior

In src/futures.rs#L818-L840, when select2() resolves:

1. All losers have their source_event_id added to cancelled_source_ids (so their completion doesn't block FIFO ordering)
2. Only activities get provider-side cancellation:

// If the loser is an activity, request provider-side cancellation.
// Timers/external/sub-orchestrations don't have worker-queue entries.
if matches!(&child.kind, Kind::Activity { .. }) {
    inner.cancelled_activity_ids.insert(source_id);
}

Impact

Aspect	Current Behavior
Completion skipped	✅ Loser's completion is skipped in FIFO ordering
Child continues running	⚠️ Sub-orchestration keeps running as orphan
No cancellation signal	⚠️ No CancelInstance sent to child
Storage cleanup	⚠️ Child instance remains in database forever

Example

let child = ctx.schedule_sub_orchestration("SlowChild", input);
let timeout = ctx.schedule_timer(Duration::from_secs(30)).into_timer();

// If timer wins, SlowChild continues running indefinitely!
let (winner, _) = ctx.select2(child, timeout).await;

Expected Behavior

When a sub-orchestration loses a select2()/select() race, the runtime should automatically send a CancelInstance work item to cancel the child orchestration, similar to how activities are cancelled via lock stealing.

Proposed Solution

1. Track sub-orchestration losers similar to cancelled_activity_ids
2. In execution.rs, generate WorkItem::CancelInstance for sub-orchestration losers
3. Enqueue these cancellation items to the orchestrator queue

Related

• Activity cancellation already works via cancelled_activity_ids and provider lock stealing
• Cascading cancellation already exists when a parent is explicitly cancelled (via client.cancel_instance())
• See proposals/auto-pruning.md which documents this as a known limitation

Acceptance Criteria

• Sub-orchestration losers in select2()/select() are automatically cancelled
• Cancellation cascades to grandchildren
• Add test: select2 with sub-orchestration vs timer, verify child is cancelled
• Update documentation to reflect automatic cancellation behavior