Prepare the patina_boot upstream RFC re-proposal

Parent roadmap: #190

Goal

Re-propose patina_boot for upstream Patina acceptance using the completed readiness evidence and a deliberately bounded API and compatibility claim.

Deliverables

• revised RFC separating generic boot mechanism from platform and OEM policy
• explicit supported-semantic contract linked to #196 and its implementation
• security model linked to #191-#194
• portability evidence linked to #118 and #158
• reproducible performance appendix linked to #108 and #195
• migration and compatibility notes for existing Patina consumers

Acceptance criteria

• every compatibility, security, and performance claim links to reproducible evidence
• unresolved gaps are listed rather than implied complete
• the upstream ask excludes Maa-specific policy
• maintainer feedback from the previous RFC is mapped to a resolution or explicit design decision
• the RFC is submitted only after the upstream-ready definition in #190 is satisfied

Dependencies

• Blocked by #191-#196, #118, and #158
• #114 consumes the accepted upstream positioning for the separate OEM package

Start with the parent roadmap in #190 and confirm its upstream-ready definition before reviewing the blocked issues #191-#196, #118, and #158. Use their completed evidence and the previous RFC feedback to assemble the bounded proposal. Done means every claim links to reproducible evidence, gaps and design decisions are explicit, Maa-specific policy is excluded, and the upstream ask is submitted only after #190 is satisfied.