Model cache: a thinned model keeps the check mode only as the model records it

SavedModel::thinned wrote `"validation": null` into the records of a model
that records no check mode, so a thinned model was never the model it came
from, even with room for every state, and its manifest grew by that key
(design::tests::a_thinned_model_keeps_the_final_state_then_the_slowest_that_fit,
a_model_cache_too_big_for_the_file_keeps_fewer_states). It copies the mode
when there is one and adds nothing otherwise; the tests' timeline model records
one, as every saved model does now, and a thinned model is asserted to keep it.

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

crates/doc/src/model_cache.rs

@@ -538,11 +538,15 @@ impl SavedModel {
             };
             out_features.push(f);
         }
-        let records = json!({
+        let mut records = json!({
             "features": out_features, "states": out_states, "model_bodies": out_bodies, "names": kept_of(names.as_slice(), &names_used),
             "sketches": kept_of(sketches.as_slice(), &sketches_used), "sheets": sheets,
-            "validation": self.records.get("validation").cloned().unwrap_or(Json::Null),
         });
+        // The check mode the results were evaluated under, as the model records it (a model
+        // that records none stays without one: adopting it is refused either way).
+        if let (Some(validation), Some(out)) = (self.records.get("validation"), records.as_object_mut()) {
+            out.insert("validation".into(), validation.clone());
+        }
         let kept: Vec<usize> = (0..self.bodies.len()).filter(|i| kernel_used.get(*i) == Some(&true)).collect();
         let bodies = kept.iter().filter_map(|i| self.bodies.get(*i).cloned()).collect();
         Some((SavedModel { records, bodies }, kept, used))

crates/io/src/design.rs

@@ -1087,7 +1087,10 @@ mod tests {
             .map(|(i, ms)| json!({"id": i + 1, "fingerprint": "0000000000000000", "ms": ms, "skipped": false, "state": i}))
             .collect();
         solidcraft_doc::SavedModel {
-            records: json!({"features": features, "states": states, "model_bodies": records, "names": [{"faces": [], "info": []}], "sketches": [], "sheets": []}),
+            records: json!({
+                "features": features, "states": states, "model_bodies": records, "names": [{"faces": [], "info": []}], "sketches": [],
+                "sheets": [], "validation": "each_feature",
+            }),
             bodies,
         }
     }
@@ -1113,6 +1116,7 @@ mod tests {
         let (states, kept, all, within) = thinned(40);
         assert_eq!((states, kept.len(), within), (vec![true; 4], 40, true));
         assert_eq!(all.records, model.records);
+        assert_eq!(thinned(16).2.records["validation"], "each_feature", "the check mode it was evaluated under");
         // The final state and the slowest other (50 ms): the shared bodies count once, else
         // these two states would need 32 bodies' room.
         for bodies in [24, 31] {