feat(core): implement missing signing host fn

[pgherveou]

Summary

• implement direct signing-host RFC-0004 alias and Bandersnatch ring-VRF proof generation
• mirror Nova full/lite person-key derivation and membership selection
• resolve Members pallet storage at one pinned finalized block, including ring pages, exponent, index, and revision
• use the Parity verifiable revision shared by the mobile implementation

Reference implementations:

• https://github.com/paritytech/polkadot-app-ios-v2/pull/1218
• https://github.com/novasamatech/triangle-js-sdks/pull/224

Validation

• cargo +nightly fmt --check
• RUSTFLAGS="-D warnings" cargo build --workspace --all-targets --all-features
• cargo +nightly clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features
• RUSTFLAGS="-D warnings" cargo check -p truapi-server --target wasm32-unknown-unknown
• cargo deny check licenses

[pgherveou]

we should cleanup this inconsistency post-merge