Meta-transactions with gas keys (DelegateV2)

[r-near]

Background. NEP-611 (gas keys) adds a second meta-transaction variant, Action::DelegateV2, so that a gas key can relay a delegate action. It mirrors the NEP-366 DelegateAction the app already signs, but the delegated payload is versioned and its nonce is a TransactionNonce — the same versioned nonce as TransactionV1 — so the signer can point at either an access key's nonce or one of a gas key's parallel nonce slots. Signatures use the NEP-461 domain-separated hashing scheme, and V2 gets a different prefix (NEP-611) from V1 (NEP-366), so the two payloads can't be confused.

Wire shape (nearcore core/primitives/src/action/delegate.rs):

• Action::DelegateV2 — action discriminant 14 (V1 Delegate is 8), carrying VersionedSignedDelegateAction { delegate_action, signature }.
• VersionedDelegateActionPayload::V2(DelegateActionV2) = 0 — versioned wrapper; unknown version tags must be rejected.
• DelegateActionV2 { sender_id, receiver_id, actions: Vec<NonDelegateAction>, nonce: TransactionNonce, max_block_height, public_key } — identical to the V1 DelegateAction except nonce is a TransactionNonce instead of a plain u64.

Current behavior. The app signs NEP-366 delegate actions through a dedicated flow (SignMode::NEP366DelegateAction, INS_SIGN_NEP366_DELEGATE_ACTION, parsing/types/nep366_delegate_action/). There is no V2 path: the versioned payload, the TransactionNonce, and the NEP-611 signing prefix are all unhandled, and action tag 14 is unknown to the action parser.

Work.

• Add a DelegateV2 signing flow mirroring the NEP-366 one, hashing the payload under the NEP-611 domain prefix (not NEP-366).
• Parse VersionedDelegateActionPayload: accept V2 (tag 0), reject unknown version tags.
• Parse the versioned TransactionNonce: a plain access-key nonce, or a gas-key { nonce, nonce_index } selecting a parallel nonce slot. Shares the versioned-nonce parsing with the gas-keys / TransactionV1 work.
• Reuse the existing delegate field parsing (sender, receiver, actions, max block height, public key) and the nested-delegate rejection.
• Display: label it a meta-transaction (delegate), show receiver + action list, and — when a gas-key nonce is used — the nonce index.
• (if the app parses relayer-side txs) accept Action::DelegateV2 (tag 14) in the transaction action parser.

Acceptance. A DelegateActionV2 payload parses and signs with the correct NEP-611 prefix; the versioned nonce (both plain and gas-key forms) is handled; unknown payload versions are rejected; tests cover a V2 delegate with both nonce forms.

Reference. near-cli-rs already carries a SignedDelegateActionV2 variant on the client side (src/transaction_signature_options/display/mod.rs) — useful prior art for the wire format.

Depends on: #50 (gas keys) — shares the TransactionNonce parsing. Related: #42 (versioned TransactionV1).

Status note: gated behind the nightly gas-keys protocol feature (protocol v85), not yet on mainnet — P1, batched with gas keys for the consolidated audit.