Coin purses (RFC 17) in the Rust core

[valentunn]

Part of Coinage R3 (#968).

Goal

Products manage coins through purses: the RFC 17 CoinPayment API in truapi-server is backed by the Rust coinage implementation, with the CoinPayment permission and consent on both platforms. Existing coins already derive under RFC 17's main-purse path (//coinage//4294967295//…), so no re-keying is needed.

Open questions

• RFC 17 has no durability guarantees yet: create_cheque, deposit and refund lack idempotency and a way to list or resume earlier attempts. It needs the same revision RFC 6 received.
• Reinstall and backup recovery of purses (purse metadata, receive keys, in-flight operations) is not covered by RFC 17 and is a separate, large topic.
• Other RFC 17 gaps: cheque encryption, the refund return path, and the clearing reference for payments made of several transactions.
• Keeping purses separate may weaken recycling privacy.

Blocked by #969 (coinage port) and #965 (shared purse logic).