Optional export, parallel to pluginOptions. Given a subscription's criteria (or a listing URL), returns the candidate works this source currently offers.

Absence is normal: a plugin without this export simply cannot back a subscription, and the host excludes it from the sources a user may pick — stating why, rather than accepting the subscription and failing at its first check.

This is the only capability that finds things. execMetadata and execDownload both act on one already-known item, which is why a subscription needs something new rather than a new way of calling them.

Declare it as export function discover(criteria: DiscoveryCriteria): Promise<DiscoveryResult>.