Concurrent PSBT spec

[yuval-block]

This is a draft BIP, extending BIP 370 with more concurrency friendly semantics

TODO

• move to bip-drafts subdir
• describe generic G-set fields using idempotence ID for keydata
• spin off different sections into dependent docs
  • explicit fee contributions G-set (one approach for defining termination conditions)
  • payment confirmations (can be in band or out of band)
  • lightning open2 / interactive-tx compatibility
    • optional removal support (G-set of tombstones)
    • explicit sort keys? that plus removal can be
  • security properties of the deterministic seed as a commitment
  • optional fields/BIPs spec that can be used to signal when these auxiliary proposals should be in effect

[yuval-block]

A related spec that can be written: G-set based in band CISA full agg - there are specific safety considerations for CISA so this needs to be thought through carefully, but in principle all of the CISA related protocol messages can be accumulated in a G-set

https://groups.google.com/g/bitcoindev/c/TF5mPfy58RQ/m/vAk1Mfg2AwAJ