Add comprehensive tests and documentation for child calls

Tests (ChildCallServiceTest.php):
- Child call scheduling with parent-close policies
- Reference resolution after child start
- Outcome recording (completed, failed, cancelled, terminated, abandoned)
- Parent-close policy enforcement (abandon, request_cancel, terminate)
- Continue-as-new child transfer
- Open/terminal children queries
- Lineage tracking across continue-as-new

Documentation (workflow-child-calls-architecture.md):
- Database schema and lifecycle state machine
- Parent-close policies (abandon, request_cancel, terminate)
- Continue-as-new semantics and child transfer
- Reference resolution (requested vs resolved IDs)
- Service API and integration points
- Query patterns and performance considerations

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: durable-workflow-ops