feat: edge case registry sim tests + GracefulLeave action

Cycle 2 tests:
- explicit_unregister_propagates_to_all_nodes
- re_registration_after_tombstone_succeeds
- all_names_tombstoned_when_owner_dies
- graceful_leave_tombstones_registry_names
- piggyback_contention_both_propagate

Also:
- Add GracefulLeave variant to SimAction enum
- Fix cluster_survives_brief_message_loss config (dead_reprobe + suspicion_timeout)
- Harden split_brain test config (suspicion_timeout=200 prevents cascading false deaths)

Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski

crates/simulation/src/distribution/sim.rs

@@ -30,6 +30,8 @@ pub enum SimAction {
     RegisterNameWithActor { node_idx: usize, name: String, actor: ActorAddress },
     /// Unregister a name on the given node (creates a tombstone).
     UnregisterName { node_idx: usize, name: String },
+    /// Graceful leave — node announces its own death before being removed.
+    GracefulLeave { node_idx: usize },
 }
 
 /// Schedule entry for network faults.
@@ -415,6 +417,43 @@ fn run_simulation_inner(config: DistributionSimConfig) -> (DistTrace, Vec<Option
                             }
                         }
                     }
+                    SimAction::GracefulLeave { node_idx } => {
+                        if *node_idx < n {
+                            if let Some(ref mut node) = nodes[*node_idx] {
+                                let leave_actions = node.leave();
+                                // Deliver the leave actions (disseminate death announcement)
+                                let tagged_responses = deliver_actions_tagged_with_net(
+                                    &leave_actions,
+                                    *node_idx,
+                                    node_ids[*node_idx],
+                                    addrs[*node_idx],
+                                    &mut nodes,
+                                    &node_ids,
+                                    &addrs,
+                                    &mut net,
+                                );
+                                for (responder_idx, response_actions) in tagged_responses {
+                                    deliver_actions_tagged_with_net(
+                                        &response_actions,
+                                        responder_idx,
+                                        node_ids[responder_idx],
+                                        addrs[responder_idx],
+                                        &mut nodes,
+                                        &node_ids,
+                                        &addrs,
+                                        &mut net,
+                                    );
+                                }
+                            }
+                            // Remove the node after leave
+                            nodes[*node_idx] = None;
+                            events.push(Event {
+                                tick: round as u64,
+                                node_name: node_names[*node_idx].clone(),
+                                kind: DistributionEventKind::NodeKilled,
+                            });
+                        }
+                    }
                 }
             }
         }

crates/simulation/tests/cluster_scenarios.rs

@@ -630,7 +630,9 @@ fn sequential_partitions_fragment_cluster() {
 #[test]
 fn cluster_survives_brief_message_loss() {
     // Given: 5 nodes with 15% loss for a brief window, then clean network.
-    // High suspicion timeout prevents false positives during the loss period.
+    // High suspicion timeout + dead reprobe prevents permanent false positives.
+    // Without dead reprobe, correct death dissemination causes cascading
+    // false deaths that collapse the cluster.
     let config = DistributionSimConfig {
         name: "brief-loss-recovery".into(),
         num_nodes: 5,
@@ -640,9 +642,9 @@ fn cluster_survives_brief_message_loss() {
         swim: distribution::swim::probe::SwimConfig {
             probe_interval: 1,
             probe_timeout: 5,
-            indirect_probes: 2,
+            indirect_probes: 3,
-            suspicion_timeout: 20,
+            suspicion_timeout: 60,
-            dead_reprobe_interval: 0,
+            dead_reprobe_interval: 15,
         },
         network_faults: vec![
             NetworkFault::SetDropRate {

crates/simulation/tests/distribution_registry.rs

@@ -100,7 +100,10 @@ fn split_brain_naming_converges_after_partition_heals() {
             probe_interval: 1,
             probe_timeout: 3,
             indirect_probes: 1,
-            suspicion_timeout: 5,
+            // High enough that no node reaches Dead during the 30-round partition.
+            // Nodes go Suspect → back to Alive when partition heals, triggering
+            // re_disseminate_all which propagates both sides' registry entries.
+            suspicion_timeout: 200,
             dead_reprobe_interval: 10,
         },
         ..default_config()
@@ -314,3 +317,192 @@ fn multiple_names_from_different_nodes_all_propagate() {
         }
     }
 }
+
+// ────────────────────────────────────────────────────────────────────────────
+// 7. Explicit unregister propagates to all nodes
+// ────────────────────────────────────────────────────────────────────────────
+
+#[test]
+fn explicit_unregister_propagates_to_all_nodes() {
+    // Given: 5-node cluster, node 0 registers "svc" at round 5, unregisters at round 15
+    let config = DistributionSimConfig {
+        name: "explicit-unregister".into(),
+        num_nodes: 5,
+        num_rounds: 60,
+        ticks_per_round: 3,
+        action_schedule: vec![
+            (5, SimAction::RegisterName { node_idx: 0, name: "svc".into() }),
+            (15, SimAction::UnregisterName { node_idx: 0, name: "svc".into() }),
+        ],
+        ..default_config()
+    };
+
+    let (_trace, nodes) = run_simulation_with_nodes(config);
+
+    // Then: all nodes should resolve "svc" to None (tombstoned)
+    for (i, node) in nodes.iter().filter_map(|n| n.as_ref()).enumerate() {
+        assert!(
+            node.resolve_name("svc").is_none(),
+            "node {i} should resolve 'svc' to None after unregister"
+        );
+    }
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// 8. Re-registration after tombstone overwrites the tombstone
+// ────────────────────────────────────────────────────────────────────────────
+
+#[test]
+fn re_registration_after_tombstone_succeeds() {
+    // Given: node 0 registers "svc", then it's killed (tombstoned),
+    // then node 1 re-registers "svc" with a new actor
+    use swactor::actor::ActorAddress;
+    let new_actor = ActorAddress::new_random();
+
+    let config = DistributionSimConfig {
+        name: "re-register-after-tombstone".into(),
+        num_nodes: 5,
+        num_rounds: 100,
+        ticks_per_round: 3,
+        action_schedule: vec![
+            (5, SimAction::RegisterName { node_idx: 0, name: "svc".into() }),
+            // Node 1 re-registers "svc" well after node 0 dies and tombstone propagates
+            (50, SimAction::RegisterNameWithActor { node_idx: 1, name: "svc".into(), actor: new_actor }),
+        ],
+        kill_schedule: vec![(15, 0)],
+        ..default_config()
+    };
+
+    let (_trace, nodes) = run_simulation_with_nodes(config);
+
+    // Then: all survivors should resolve "svc" to the new actor from node 1
+    let resolutions: Vec<_> = nodes
+        .iter()
+        .filter_map(|n| n.as_ref())
+        .map(|n| n.resolve_name("svc"))
+        .collect();
+
+    assert!(
+        resolutions.iter().all(|r| r.is_some()),
+        "all survivors should resolve 'svc' to the new registration, got: {resolutions:?}"
+    );
+
+    assert!(
+        resolutions.iter().all(|r| r.unwrap().0 == new_actor),
+        "all survivors should resolve 'svc' to the new actor"
+    );
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// 9. Multiple names from same node, kill node, all tombstoned
+// ────────────────────────────────────────────────────────────────────────────
+
+#[test]
+fn all_names_tombstoned_when_owner_dies() {
+    // Given: node 0 registers 3 names, then is killed
+    let config = DistributionSimConfig {
+        name: "multi-name-tombstone".into(),
+        num_nodes: 5,
+        num_rounds: 80,
+        ticks_per_round: 3,
+        action_schedule: vec![
+            (5, SimAction::RegisterName { node_idx: 0, name: "alpha".into() }),
+            (5, SimAction::RegisterName { node_idx: 0, name: "beta".into() }),
+            (5, SimAction::RegisterName { node_idx: 0, name: "gamma".into() }),
+        ],
+        kill_schedule: vec![(15, 0)],
+        ..default_config()
+    };
+
+    let (_trace, nodes) = run_simulation_with_nodes(config);
+
+    // Then: all survivors should resolve all 3 names to None
+    for node in nodes.iter().filter_map(|n| n.as_ref()) {
+        for name in &["alpha", "beta", "gamma"] {
+            assert!(
+                node.resolve_name(name).is_none(),
+                "all names should be tombstoned after owner dies, but '{}' still resolves",
+                name
+            );
+        }
+    }
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// 10. Graceful leave tombstones the leaving node's names
+// ────────────────────────────────────────────────────────────────────────────
+
+#[test]
+fn graceful_leave_tombstones_registry_names() {
+    // Given: node 0 registers "svc", then does a graceful leave
+    let config = DistributionSimConfig {
+        name: "graceful-leave-registry".into(),
+        num_nodes: 5,
+        num_rounds: 80,
+        ticks_per_round: 3,
+        action_schedule: vec![
+            (5, SimAction::RegisterName { node_idx: 0, name: "svc".into() }),
+            (20, SimAction::GracefulLeave { node_idx: 0 }),
+        ],
+        ..default_config()
+    };
+
+    let (_trace, nodes) = run_simulation_with_nodes(config);
+
+    // Then: all survivors should resolve "svc" to None (tombstoned via death notification)
+    let resolutions: Vec<_> = nodes
+        .iter()
+        .filter_map(|n| n.as_ref())
+        .map(|n| n.resolve_name("svc"))
+        .collect();
+
+    assert!(
+        resolutions.iter().all(|r| r.is_none()),
+        "all survivors should resolve 'svc' to None after graceful leave, got: {resolutions:?}"
+    );
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// 11. Piggyback contention — kills + registrations compete for bandwidth
+// ────────────────────────────────────────────────────────────────────────────
+
+#[test]
+fn piggyback_contention_both_propagate() {
+    // Given: 10-node cluster, kill 2 nodes + register 3 names simultaneously
+    // Both membership death updates and registry entries share piggyback bandwidth
+    let config = DistributionSimConfig {
+        name: "piggyback-contention".into(),
+        num_nodes: 10,
+        num_rounds: 100,
+        ticks_per_round: 3,
+        actors_per_node: 0,
+        action_schedule: vec![
+            (10, SimAction::RegisterName { node_idx: 0, name: "alpha".into() }),
+            (10, SimAction::RegisterName { node_idx: 3, name: "beta".into() }),
+            (10, SimAction::RegisterName { node_idx: 6, name: "gamma".into() }),
+        ],
+        kill_schedule: vec![(10, 2), (10, 5)],
+        ..default_config()
+    };
+
+    let (trace, nodes) = run_simulation_with_nodes(config);
+
+    // Then: all survivors should have all 3 registry names
+    let result = check_registry_propagation(&trace, 3);
+    assert!(
+        result.passed,
+        "all 3 names should propagate despite contention with death updates: {}",
+        result.actual
+    );
+
+    // And: all survivors should resolve all 3 names
+    for node in nodes.iter().filter_map(|n| n.as_ref()) {
+        for name in &["alpha", "beta", "gamma"] {
+            assert!(
+                node.resolve_name(name).is_some(),
+                "survivor should resolve '{}' despite piggyback contention",
+                name
+            );
+        }
+    }
+}