Co-signing protocol — detached signatures, byte-identical

[iret77]

Co-signing protocol — detached signatures, byte-identical across parties. The real cross-party test of R6: do two independent stacks produce the same anchored form, or not?

Scope

• Detached-signature collection across multiple parties (ANP §6.3)
• Deterministic proof[] assembly across party boundaries
• Byte-identical anchored form verified across two independent omadia instances
• Counterparty identity exchange/discovery (what's enough to address a counterparty?)

Done

Two independent omadia instances produce a byte-identical anchored memorandum/contract; the signatures verify cross-party.

Blocked by: #4 (objects), #6 (identity), #8 (anchor).
Refs: plan.md §5.2, R6.