RFC: NFT Sending (contacts)

Description

Sending an NFT means moving it from one of my purses (#512) to a purse of the recipient. The sender picks a person from their contacts and knows nothing else about them. The recipient side is resolved without the sender learning the recipient's accounts: the item lands in the recipient's next free purse. Coinage solves the same problem for coins with receivables and invoices (RFC 0017); NFTs need the same ability.

Motivation

Currently, a product can only send NFTs between dev accounts whose derivation is public, because only the recipient's own host can derive their purses. Users cannot send an item to a real person. The Contacts API (#484) tells us who the recipient is; nothing tells us where to send the item.

Requirements

• Send an NFT to a picked contact without knowing any of their addresses.
• The sender learns at most one receiving account and nothing else about the recipient's purses.
• Sending works when the recipient is offline; delivery cannot require both people to be online at once.
• The item appears on the recipient's shelf without manual setup on their side.
• Same behaviour on every host (desktop, mobile, web).