fix(simulation): fix triangle state save/load and persist contamination

Three save/load bugs fixed:
- ContaminationActive not persisted in SaveStateV1 — caused double-fire
  of contamination pressure on reload after tick 300.
- Loaded triangle entities missing ActiveSim marker — made them
  invisible to escalation and contamination systems after any load.
- Existing triangle entities not despawned before load — created
  duplicates, doubling tension escalation per tick.

Also: HashSet → BTreeSet for D-010 compliance, defensive event queue
reset on load, and three regression tests for triangle roundtrip.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/simulation/save_state.rs

@@ -105,6 +105,12 @@ pub struct SaveStateV1 {
     /// DLC can populate it without a save format migration.
     #[serde(default)]
     pub modifications: Vec<Modification>,
+    /// Whether contamination has already activated (#254).
+    /// Persisted to prevent double-firing on save/load — without this,
+    /// reloading a save after tick 300 would re-trigger contamination
+    /// and apply a duplicate tension delta to all ActiveFork triangles.
+    #[serde(default)]
+    pub contamination_active: bool,
 }
 
 /// Per-NPC state snapshot for `SaveStateV1`.
@@ -418,6 +424,7 @@ mod tests {
             triangle_states: vec![],
             open_doors: vec![],
             modifications: vec![],
+            contamination_active: false,
         }
     }
 
@@ -816,6 +823,7 @@ mod tests {
             triangle_states: vec![],
             open_doors: vec![],
             modifications: vec![],
+            contamination_active: false,
         };
 
         let bytes = save.to_bytes().expect("serialize");