[RFC] Define the RelayOS threat model and protocol v1

[rufatixx]

Outcome

Write a reviewable threat model and a versioned protocol-v1 specification before adding a physical transport.

Scope

• assets, attackers, trust boundaries, and metadata exposure
• recipient-key discovery and verification assumptions
• authenticated and unauthenticated packet fields
• replay, clock-skew, denial-of-service, and compromised-recipient cases
• compatibility rules for packet format changes
• explicit non-goals for the alpha

Acceptance criteria

• A document is added under docs/.
• Every current cryptographic claim in the README maps to the document.
• Open questions and decisions are clearly separated.
• The proposal is reviewed before implementation changes begin.

This issue is design work. It must not claim that RelayOS has been audited.