implement optional stale-while-revalidate behavior

To Do: implement optional stale-while-revalidate behavior

Why:
- support & maximize progressive behavior
- allow stale or placeholder data to be shown optimistically while new data is loaded in the background, ideally in a non-blocking fashion

Acceptance Criteria:
- `IDBSideloader` constructor accepts an optional prop to enable stale-while-revalidate (swr) behavior
- swr behavior attempt to load cached data, and then re-fetches data in the background
- swr option is compatible with web worker option

example: `IDBSideloader` constructor prop
```ts
const loader = new IDBSideloader({ swr: true });
```

example: per-asset basis
```html

```

Optional:
- implement similar API's & behaviors: https://swr.vercel.app/docs/api

see: https://www.mnot.net/blog/2007/12/12/stale

Start at the IDBSideloader constructor and trace the existing cached-data loading and web worker paths. Implement the optional swr constructor and per-asset behavior described in the issue, ensuring cached data is shown before background re-fetching and that the web worker option remains compatible. Done means both constructor and data-swr usage work as specified.

Written by the indexing model from the issue text.