docs: add EIP-712 signTypedData interfaces and docs (WALLET-1253)

[ost-ptk]

Summary

Documents the EIP-712 typed-data signing feature in the Casper Wallet SDK README, so dApp developers have the interfaces and usage docs for signTypedData. Docs-only — no runtime code changes.

Part of epic WALLET-1250 (EIP-712 signature requests). Interfaces taken verbatim from the feature's source definitions in casper-click-websdk (isdk.ts, PR #155).

Closes WALLET-1253.

What's added (README.md)

• Methods — signTypedData(params: SignTypedDataParams, signingPublicKey: string): Promise<SignTypedDataResult | undefined> with params/returns notes and a usage example, matching the style of the existing sign / signMessage entries.
• Types — SignTypedDataParams, SignTypedDataResult (and EIP712HashArtifacts).
• CasperWalletSupports — adds signTypedDataEIP712 = 'sign-typed-data-eip712' to the supports enum, and references it from getActivePublicKeySupports. (Forward-looking: the canonical value is defined in the SDK types; documented as part of the EIP-712 rollout.)
• Error Handling — signTypedData error codes section documenting SignTypedDataErrorCodes (7 codes) and that signTypedData returns failures in the result.error / result.errorCode envelope rather than throwing.

Notes

• No changes to index.js or any runtime code — README only.
• prettier --check passes (repo .prettierrc: single quotes, arrowParens: avoid, trailingComma: none).
• The SDK's SignTypedDataErrorCodes is a superset of the 3 validation codes in casper-wallet-core: the extra 4 (USER_REJECTED, SIGNATURE_SCHEME_NOT_SUPPORTED, ACCOUNT_NOT_FOUND, NOT_AUTHORIZED) originate at the wallet/transport layer, not in the pure logic library.

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