expose pipeline promise on savezip

Author: bencmbrook

src/api.ts

@@ -192,6 +192,7 @@ async function save(
     });
     await writer.write(...files);
     await writer.close();
+    await writer.done();
     return;
   }
 

tests/api.test.ts

@@ -479,6 +479,7 @@ describe('Penumbra API', () => {
       })),
     );
     await writer.close();
+    await writer.done();
     assert.isTrue(
       progressEventFiredAndWorking,
       'zip progress event fired & emitted expected properties',