Motivation

• Capture and formalize the design decision that NetEngine worlds treat Authority (naming, numbering, trust, identity, transit, mail, and service discovery) as first-class governance institutions rather than incidental configuration.
• Make explicit why gateway modes represent high-level boundary postures, why federation is import/export of authority (not only connectivity), and why peering imports reachability only.
• Provide guidance that persistent/self-hosted authority state is important for long-term continuity but may be deferred for MVP so long as the domain model preserves authority distinctions and future migration paths.

Description

• Add a new ADR at docs/adr-authority-autonomous-internet-runtime.md that defines the "authority-autonomous internet runtime" model, rationale, consequences, and open questions.
• Document why gateway modes are policy postures, why federation must be modeled as explicit authority import/export, why peering conveys reachability-only, and why default in-world institutions should exist.
• Link the new ADR from the top of docs/decisions.md under a new "Related design notes" section so the architecture decisions index references the note.

Testing

• Ran git diff --check to validate whitespace/patch issues and it produced no errors.
• Verified the new ADR content and the link insertion by inspecting docs/adr-authority-autonomous-internet-runtime.md and docs/decisions.md.
• No unit or integration tests were added for this documentation change and none were executed as part of this PR.

Codex Task