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/tests/environmental_interaction.rs

@@ -419,6 +419,7 @@ fn minimal_save() -> SaveStateV1 {
         triangle_states: vec![],
         open_doors: vec![],
         modifications: vec![],
+        contamination_active: false,
     }
 }
 

server/tests/information_boundaries.rs

@@ -207,6 +207,7 @@ fn save_state_npc_kg_isolation() {
         triangle_states: vec![],
         open_doors: vec![],
         modifications: vec![],
+        contamination_active: false,
     };
 
     // Roundtrip: serialize → deserialize.