Simplify replaySafeHash to the 8130 non-EIP-712 signed-message convention

[amiecorso]

Summary

Simplify AccountConfiguration.replaySafeHash from a full EIP-712 digest to the 8130 non-EIP-712 signed-message convention, binding a 1271 signature to account + chainId under a fixed typehash.

Motivation

replaySafeHash was the lone EIP-712 signed-message digest in AccountConfiguration. Every other signed message (ActorInitialization/import, SignedActorChanges, SignedLockChange) is deliberately non-EIP-712 "to mitigate eth_signTypedData phishing." This aligns the ERC-1271 wrap with that house convention and removes now-redundant machinery.

• Binds using only account + chainId (verifyingContract already uniquely identifies the account; name/version added nothing).
• The fixed typehash provides domain separation; the deliberate absence of 0x1901 keeps eth_signTypedData from producing these digests (anti-phishing), matching the other signed-message typehashes.
• Cross-account replay closed by account; cross-chain replay closed by block.chainid.

Changes

• Replace the EIP-712 replaySafeHash with keccak256(abi.encode(SIGNED_MESSAGE_TYPEHASH, account, block.chainid, hash)).
• Add SIGNED_MESSAGE_TYPEHASH = keccak256("SignedMessage(address account,uint256 chainId,bytes32 hash)").
• Remove now-unused _EIP712_DOMAIN_TYPEHASH, _PERSONAL_SIGN_TYPEHASH, _ACCOUNT_DOMAIN_NAME_HASH, _ACCOUNT_DOMAIN_VERSION_HASH, and _accountDomainSeparator.
• verifySignature and DefaultAccount.isValidSignature are unchanged (the account already forwards to verifySignature).

Out of scope / decisions

• ERC-7739 TypedDataSign (readable signatures) intentionally not added; it is an account-layer concern, not core, and its readability benefit is marginal for passkey-backed / first-party-curated accounts.
• A "replayable wrapper" (wildcard account/chainId) was considered and declined: it inverts the containment the wrap exists for, and can be added later at the account layer via authenticateActor if a concrete use case arises.

Testing

• forge build clean.
• forge test — all 325 unit tests pass. Tests derive digests from the contract's replaySafeHash, so behavior adapts automatically to the new formula.