Add comprehensive YAML deserialization test for Topology struct, verifying:
- Deserialization from plan §4 YAML format (RG=2, 6 nodes, RF=2)
- Correct topology properties (shards, rf, replica_group_count)
- groups() iterator returns groups in ascending order
- Each group holds exactly its configured nodes
- Node addresses, replica groups, and statuses are correct
All 41 topology tests pass, covering:
- State machine transitions (legal and illegal)
- Write eligibility rules per status
- Group and node iteration
- Healthy node filtering
- YAML deserialization
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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