## Dynamic Worker fails to resolve relative imports between modules in a subdirectory

### Reproduction

The following code works on Cloudflare but fails on celld:

```js const dyn = env.LOADER.load({ compatibilityDate: '2026-01-01', mainModule: 'dir/a.js', modules: { 'dir/a.js': ` import { x } from './b.js' export default { fetch() { return new Response(String(x)) } } `, 'dir/b.js': `export const x = 55`, }, })

return dyn.getEntrypoint().fetch(new Request('http://example.com')) ```

### Expected behavior

HTTP 200 with response body `55`, as observed on Cloudflare.

### Actual behavior

The request fails with:

``` Worker failed: rejected: Error: instantiate: <none> ```

The celld logs contain:

``` WARN celld::js::modules: resolve: no stub for specifier spec=./b.js ```

### Additional context

The issue appears to be relative module resolution: `./b.js` should resolve relative to the importing module (`dir/a.js`), yielding `dir/b.js`, which is present in the supplied module map.