Add Linux interoperability tests and preview documentation

[sij411]

Description

Validate the Linux preview against compatible ActivityPub implementations and document its supported workflow and limitations.

Scope

• Add an end-to-end Linux runtime test covering discovery, Follow, Accept, local Note creation, and Create(Note) delivery.
• Test against at least one real or controlled compatible implementation, such as a Fedify example, snac2, Mastodon, or a dedicated test inbox.
• Document configuration, startup, local account discovery, note creation, and troubleshooting.
• Document supported activities, partial JSON-LD behavior, security assumptions, and known limitations.
• Provide a repeatable interoperability checklist for contributors.

Done when

• A user can follow the documentation to run the single-user Linux preview.
• WebFinger and actor discovery work externally.
• A real signed Follow produces an Accept.
• A locally created Note is delivered as Create(Note) to at least one compatible ActivityPub inbox.
• Automated and manual interoperability results are recorded.

Out of scope

• Production-readiness claims.
• Broad server compatibility guarantees.
• Embedded runtime instructions.