P1.2 Topology type + node state machine - Add YAML deserialization test

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>

crates/miroir-core/src/topology.rs

@@ -818,11 +818,105 @@ mod tests {
         assert!(msg.contains("draining"));
     }
 
+    // --- Plan §4 YAML deserialization test ---
+    // This test verifies that a Topology can be deserialized from YAML
+    #[test]
+    fn test_topology_deserialize_from_yaml() {
+        // YAML matching plan §4 example structure (RG=2, 6 nodes, RF=2)
+        let yaml = r#"
+shards: 64
+rf: 2
+groups:
+  - id: 0
+    nodes:
+      - "meili-0"
+      - "meili-1"
+      - "meili-2"
+  - id: 1
+    nodes:
+      - "meili-3"
+      - "meili-4"
+      - "meili-5"
+nodes:
+  meili-0:
+    id: "meili-0"
+    address: "http://meili-0.search.svc:7700"
+    replica_group: 0
+    status: Active
+  meili-1:
+    id: "meili-1"
+    address: "http://meili-1.search.svc:7700"
+    replica_group: 0
+    status: Active
+  meili-2:
+    id: "meili-2"
+    address: "http://meili-2.search.svc:7700"
+    replica_group: 0
+    status: Active
+  meili-3:
+    id: "meili-3"
+    address: "http://meili-3.search.svc:7700"
+    replica_group: 1
+    status: Active
+  meili-4:
+    id: "meili-4"
+    address: "http://meili-4.search.svc:7700"
+    replica_group: 1
+    status: Active
+  meili-5:
+    id: "meili-5"
+    address: "http://meili-5.search.svc:7700"
+    replica_group: 1
+    status: Active
+"#;
+
+        let topology: Topology = serde_yaml::from_str(yaml).expect("Failed to deserialize topology from YAML");
+
+        // Verify topology properties
+        assert_eq!(topology.shards(), 64, "S should be 64");
+        assert_eq!(topology.rf(), 2, "RF should be 2");
+        assert_eq!(topology.replica_group_count(), 2, "RG should be 2");
+
+        // Verify groups() iterator returns RG groups in ascending order
+        let groups: Vec<_> = topology.groups().collect();
+        assert_eq!(groups.len(), 2, "Should have 2 groups");
+        assert_eq!(groups[0].id, 0, "First group should be ID 0");
+        assert_eq!(groups[1].id, 1, "Second group should be ID 1");
+
+        // Verify each group holds exactly its configured nodes
+        let group0_nodes = groups[0].nodes();
+        assert_eq!(group0_nodes.len(), 3, "Group 0 should have 3 nodes");
+        assert!(group0_nodes.contains(&NodeId::new("meili-0".to_string())));
+        assert!(group0_nodes.contains(&NodeId::new("meili-1".to_string())));
+        assert!(group0_nodes.contains(&NodeId::new("meili-2".to_string())));
+
+        let group1_nodes = groups[1].nodes();
+        assert_eq!(group1_nodes.len(), 3, "Group 1 should have 3 nodes");
+        assert!(group1_nodes.contains(&NodeId::new("meili-3".to_string())));
+        assert!(group1_nodes.contains(&NodeId::new("meili-4".to_string())));
+        assert!(group1_nodes.contains(&NodeId::new("meili-5".to_string())));
+
+        // Verify node addresses are correct
+        let node0 = topology.node(&NodeId::new("meili-0".to_string())).unwrap();
+        assert_eq!(node0.address, "http://meili-0.search.svc:7700");
+        assert_eq!(node0.replica_group, 0);
+        assert_eq!(node0.status, NodeStatus::Active);
+
+        let node5 = topology.node(&NodeId::new("meili-5".to_string())).unwrap();
+        assert_eq!(node5.address, "http://meili-5.search.svc:7700");
+        assert_eq!(node5.replica_group, 1);
+        assert_eq!(node5.status, NodeStatus::Active);
+    }
+
     // --- Plan §4 YAML example test ---
     // This test verifies that a Topology can be correctly built from the plan §4 YAML
     // example structure (RG=2, 6 nodes, RF=2)
     #[test]
     fn test_topology_from_plan_section_4_yaml_structure() {
+        // Plan §4 YAML example:
+        // replica_groups: 2
+        // shards: 64
+        // replication_factor: 2
         // Plan §4 YAML example:
         // replica_groups: 2
         // shards: 64