Construct the frozen source3 migration envelope explicitly

crates/oa-core/src/language.rs

@@ -1,6 +1,7 @@
 //! Typed OA/SA language, tolerant source analysis, and explicit V1/V2/V3/V4 migration.
 #[path = "language/semantic_source.rs"]
 mod semantic_source;
+mod source3_bridge;
 // Staged syntax foundation only. Public source8 admission waits for complete
 // declaration lowering, source updates and the root-owned resource gate.
 #[cfg(test)]
@@ -1834,10 +1835,7 @@ pub(crate) fn resolve_declarations(
         target.nodes.insert(node.id, node);
     }
     if legacy {
-        let mut value = serde_json::to_value(&project).expect("project serialization");
-        value["elements"] = Value::Object(legacy_elements);
-        let migrated = serde_json::from_value(value)
-            .map_err(crate::persistence::FileError::Json)
+        let migrated = source3_bridge::from_scratch(project, legacy_elements)
             .and_then(|old| crate::persistence::v3::migrate(old, limits))
             .map_err(|error| {
                 let issue = if let crate::persistence::FileError::Model { issues, .. } = &error {