Build a runnable single-user ActivityPub server runtime on top of the portable
Feder core.

Done when a user can expose an actor through WebFinger, receive a signed
Follow, send an Accept, create a Note locally, and deliver
Create(Note) to at least one compatible ActivityPub inbox.

The initial implementation and testing may focus on Linux, but the runtime
should avoid unnecessary operating-system-specific assumptions so that macOS
and Windows support can be added or confirmed later.

Production hardening, broad ActivityStreams coverage, and embedded runtimes
remain out of scope.