Claim transfers

Real-world ownership changes. The registry entry can move between wallets via a two-party signed handoff, and the current-holder pointer updates without any on-chain token movement:

1. Current holder signs a “Verity — transfer claim” message naming the destination wallet.
2. Destination wallet signs a “Verity — accept transfer” message.
3. Server verifies both signatures. The listing's current_holder_wallet updates.
4. A status update is appended to the update log.
5. Optionally, either party can anchor the transfer on-chain — see next section.

The transfer can be cancelled by the outgoing wallet or rejected by the incoming wallet at any time before acceptance. Only one pending transfer per asset is allowed.