Bandersnatch ring-VRF provider trait.
Defines the VrfProvider interface for ring-VRF operations used in
Polkadot lite-person attestation. The trait carries no cryptographic
dependencies — implementations live in separate backend crates
(e.g., host-vrf-native for arkworks, or platform-specific bindings).
The attestation flow itself (extrinsic construction, People chain submission) is out of scope for this crate. See the backlog for tracking.