spec: add EIP-712 signing scheme for zone transactions

[dankrad]

Summary

Adds an alternate signing scheme for zone transactions, based on EIP-712 typed messages, so that wallets without native Tempo or zone envelope support (hardware wallets, WalletConnect-only dapps, legacy SDKs) can author zone transactions by signing structured data with eth_signTypedData_v4.

• Introduces signature type 0x05 layered onto the existing Tempo transaction envelope. The wire-level envelope and fields are unchanged; only the digest the signature commits to changes (EIP-712 digest instead of the native tx hash).
• Domain separator binds chainId (which already encodes zoneId) and the zone's ZonePortal address, giving each zone a unique domain and a stable address for wallets to display.
• Two typed structs:
  • ZoneTransaction — used when a root account signs directly. Mapping to envelope fields is bijective.
  • AccessKeyZoneTransaction — used when a Keychain access key signs on behalf of a root. Includes userAddress as the first field so the wallet prompt shows which root account the access key is acting for.
• Access-key composition reuses the existing Keychain V2 wrapper (0x04 || innerSig || user_address) around a 0x05 inner signature. Secp256k1 access keys work under EIP-712 without re-registration (the curve is registered, not the digest scheme).
• Keychain V1 (0x03) is rejected as an EIP-712 wrapper because it does not bind user_address into the signing hash — an EIP-712 wallet cannot see user_address, so V1 wrapping would allow a malicious client to rebind the signature to any root.
• Fee-payer delegation and EIP-7702 authorization lists are intentionally not supported under this type; applications needing either use the native Tempo signing scheme.
• Private RPC > Signature Types table is extended with the new type, and the TOC is updated.

Test plan

• Spec-only change; no runtime behavior affected until the format is implemented.
• Confirm the rendered TOC entry and internal anchors (#eip-712-typed-transactions, #access-key-signing, #signature-types) resolve correctly on GitHub.
• Reviewers sanity-check the ZoneTransaction and AccessKeyZoneTransaction field sets against the final Tempo transaction envelope used on zones.

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