BRC-246: Session Upgrade (IPv6 messagebox hot path)

[GenericCPU]

Summary

Adds BRC-246: Session Upgrade — a signed, short-lived IPv6 endpoint offer that may ride in BRC-33 messagebox traffic so two online wallets can move the rest of that session onto a mutually authenticated socket.

• Messagebox stays rendezvous / offline inbox; socket is hot-path only (no custody)
• Global-unicast IPv6 only; 60s offer lifetime; lexicographically smaller identity dials
• 300 ms race with immediate fail-through to messagebox on connect errors
• Number 246 proposed (not a BSVA assignment); renumber if a lower free number is preferred at review

Implemented in HandCash Desktop (direct session server + message transport).

Test plan

• Spec review (peer / messagebox maintainers)
• Confirm number 246 is still free at merge time (or assign lowest available)
• Cross-links to BRC-33 / BRC-169 resolve
• README.md, SUMMARY.md, and peer-to-peer/README.md index entries match