PSBT Coordination

The end-to-end workflow of constructing, partially signing, relaying, and completing a PSBT across SSP's two devices. SSP Wallet initiates the PSBT by selecting UTXOs, attaching its signature, and encrypting the payload for relay; SSP Key decrypts, verifies the outputs, applies the second signature, and returns the finalized transaction. The coordination layer ensures neither device ever holds a complete transaction until both have reviewed it.