fix(simulation): validate dangling with_role in TriangleDef constraints

TriangleDef.validate() now rejects relationship constraints where
with_role references a role not in the triangle's three roles.
Catches authoring errors at load time instead of silently producing
broken constraint data.

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

server/src/content/template.rs

@@ -430,6 +430,7 @@ impl TriangleDef {
             ));
         }
 
+        let role_set: std::collections::BTreeSet<&RoleId> = self.roles.iter().collect();
         for (i, constraint) in self.relationship_constraints.iter().enumerate() {
             if constraint.required_trust.min > constraint.required_trust.max {
                 return Err(format!(
@@ -437,6 +438,12 @@ impl TriangleDef {
                     i, constraint.required_trust.min, constraint.required_trust.max
                 ));
             }
+            if !role_set.contains(&constraint.with_role) {
+                return Err(format!(
+                    "relationship_constraints[{}]: with_role '{}' not in triangle roles",
+                    i, constraint.with_role.0
+                ));
+            }
         }
 
         Ok(())