Tests: the io crate's hand-built saved models list their rounds, as format 7 records do

Model cache format 7 (f3cf0c4f3) records a `rounds` list in every saved model, and reading one
without it is refused ("saved model: rounds"), as is a model without any of its other lists:
the states, the preview and the thinned model all came back empty. The io tests' saved_model
and timeline_model wrote format-6 records, so six tests (the round trip, the repeated part
stored once, and the four too-big-for-the-file cases) read nothing. Their bodies are none made
by rounds: the list is empty, and every expectation stays as it was.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/io/src/design.rs

@@ -968,8 +968,8 @@ mod tests {
         assert!(result.is_err_and(|e| e.to_string().contains("not a regular file")));
     }
 
-    /// A saved model of one state holding `bodies` (no document evaluated it: enough for the
-    /// container, which stores whatever model it is given).
+    /// A saved model of one state holding `bodies`, none made by rounds (no document evaluated
+    /// it: enough for the container, which stores whatever model it is given).
     fn saved_model(bodies: Vec<solidcraft_kernel::Body>) -> solidcraft_doc::SavedModel {
         let records: Vec<Value> = (0..bodies.len())
             .map(|i| json!({"name": format!("Body{i}"), "feature": 1, "component": 0, "key": "0000000000000001", "names": 0, "body": i}))
@@ -982,6 +982,7 @@ mod tests {
             records: json!({
                 "features": [{"id": 1, "fingerprint": "0000000000000000", "ms": 0.0, "skipped": false, "state": 0}],
                 "states": [state], "model_bodies": records, "names": [{"faces": [], "info": []}], "sketches": [], "sheets": [],
+                "rounds": [],
             }),
             bodies,
         }
@@ -1089,7 +1090,7 @@ mod tests {
         solidcraft_doc::SavedModel {
             records: json!({
                 "features": features, "states": states, "model_bodies": records, "names": [{"faces": [], "info": []}], "sketches": [],
-                "sheets": [], "validation": "each_feature",
+                "sheets": [], "validation": "each_feature", "rounds": [],
             }),
             bodies,
         }