test: headers first stage (#7127)

crates/primitives/src/stage/id.rs

@@ -120,4 +120,11 @@ mod tests {
 
         assert!(!StageId::Execution.is_downloading_stage());
     }
+
+    // Multiple places around the codebase assume headers is the first stage.
+    // Feel free to remove this test if the assumption changes.
+    #[test]
+    fn stage_all_headers_first() {
+        assert_eq!(*StageId::ALL.first().unwrap(), StageId::Headers);
+    }
 }