fix(csv-loader): handle object foreign keys in reverse lookups

Update `csvToModule` to check if a foreign key value is an object
containing the default primary key before converting it to a string.
This ensures correct key mapping when references are resolved as
objects rather than primitive IDs.

src/csv-loader/loader.ts

@@ -1199,7 +1199,8 @@ export function csvToModule(
       reverseLookupInits.push(
         `const ${revLookupVar} = new Map();`,
         `for (const r of _raw) {`,
-        `  const k = String(r.${decl.foreignKey});`,
+        `  const kv = r.${decl.foreignKey};`,
+        `  const k = String(typeof kv === "object" && "${defaultPrimaryKey}" in kv ? kv.${defaultPrimaryKey} : kv);`,
         `  if (!${revLookupVar}.has(k)) ${revLookupVar}.set(k, []);`,
         `  ${revLookupVar}.get(k).push(r);`,
         `}`,
@@ -1209,7 +1210,8 @@ export function csvToModule(
       reverseLookupInits.push(
         `const ${revLookupVar} = new Map();`,
         `for (const r of ${varName}()) {`,
-        `  const k = String(r.${decl.foreignKey});`,
+        `  const kv = r.${decl.foreignKey};`,
+        `  const k = String(typeof kv === "object" && "${defaultPrimaryKey}" in kv ? kv.${defaultPrimaryKey} : kv);`,
         `  if (!${revLookupVar}.has(k)) ${revLookupVar}.set(k, []);`,
         `  ${revLookupVar}.get(k).push(r);`,
         `}`,