fix(director): discard an asset cancelled during reader cleanup

The finally block awaits reader.cancel(), so a cancel arriving there still let
the bytes return. Rechecks the signal after cleanup, with a test that keeps the
caller's abort reason, and wraps the current-state note.

src/director/packs/source.js

@@ -51,6 +51,8 @@ export function createAssetDirectorySource({
       await reader.cancel().catch(() => {});
       reader.releaseLock();
     }
+    // Cleanup awaits too; a cancel during it must still discard the asset.
+    signal?.throwIfAborted();
     const bytes = new Uint8Array(length);
     let offset = 0;
     for (const chunk of chunks) {