kb-fediverse-webfinger-discovery

Background knowledge about reverse WebFinger discovery — finding a WebFinger address from an ActivityPub actor — based on FEP-2c59 (DRAFT status). Covers the `webfinger` property on actors, the forward vs reverse discovery problem, why `preferredUsername` is insufficient for reverse lookups, domain mismatch use cases, validation requirements (no-redirect string comparison), security considerations, JSON-LD context registration, the community debate around alternative approaches (`alsoKnownAs`, `aliases`, property naming), Mastodon's current reverse discovery algorithm and its limitations, and the W3C SWICG acknowledgment of the problem. Load when the user asks about reverse WebFinger discovery; how to get a WebFinger address from an actor URL; the `webfinger` property on ActivityPub actors; why `preferredUsername` is not enough for constructing WebFinger handles; domain mismatch between actor URL and WebFinger address; FEP-2c59; or how Mastodon resolves actor URLs back to `user@domain` handles.