Shared purse logic for NFT and coin purses (conditional) #965

Part of #637. Builds on #964. Conditional: done only if generalizing proves worth it.

Goal

One purse engine shared by NFT purses (#512) and coin purses (RFC 17): purses as key subtrees, receive keys, cheques, the two-phase handoff (reserve, commit, release, reconcile, retract), and deposit and refund as durable operations with a clearing status stream.

Open question

How far to generalize: will coin and NFT purses (cheque format, handoff flow, API) evolve together or independently? If together, build one generic engine here. If independently, close this issue and build NFT purses directly on #964.

Risks

• It is designed before coinage is ported, so it is shaped mostly by NFTs; coin needs such as selecting by amount and making change may not fit later.
• It inherits the risk that the NFT purse assumptions turn out wrong against the actual pallet (see NFT purses in the Rust core #966).

Blocked by #964.