Add EIP-191 personal message (personal_sign) support

[alisinabh]

First PR of the v0.7 "Auth" tier (stacked chain: this → #266 universal verification → #267 SIWE).

What

• New Ethers.PersonalMessage module: hash/1 (EIP-191 version 0x45), recover/2, recover!/2, and EOA-only verify/3 (case-insensitive address compare, checksummed output).
• New optional signer callback personal_sign/2 on Ethers.Signer — named after the RPC method it mirrors — implemented for:
  • Ethers.Signer.Local (secp256k1 sign over the EIP-191 hash, v ∈ {27, 28})
  • Ethers.Signer.JsonRPC (delegates to personal_sign)
• Top-level Ethers.personal_sign/2 and personal_sign!/2, mirroring sign_typed_data/2.

Design notes

• Naming: personal_sign (rather than ethers.js/viem-style sign_message) is unambiguous about the signing scheme and matches the module (PersonalMessage) and the underlying RPC method; it also leaves clean room for future sign_* callbacks (e.g. EIP-7702 sign_authorization).
• Messages are plain binaries treated as raw bytes; a "0x..." string signs as literal text (documented) — matching ethers.js signMessage string semantics. No {:raw, ...} variant.
• Signatures accepted as 0x-hex or raw 65 bytes, v ∈ {0, 1, 27, 28}.

Testing

• TDD; 100% coverage on new/changed lines including error branches.
• Vectors ground-truthed against foundry (cast wallet sign / cast hash-message) across 2 keys and 4 message shapes (ASCII, empty, multibyte UTF-8, raw bytes), plus a live byte-for-byte Local↔JsonRPC cross-check against anvil's personal_sign.
• Full suite green; mix format, mix credo --strict, mix dialyzer clean.

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Pull request overview

This PR introduces first-class support for EIP-191 “personal message” signing (personal_sign) across the Ethers API surface (hashing/recovery/verification utilities plus signer integrations), enabling common wallet-login/auth flows.

Changes:

• Add Ethers.PersonalMessage for EIP-191 hashing, signer recovery, and signature verification.
• Add optional sign_message/2 callback to Ethers.Signer, implemented by Ethers.Signer.Local and Ethers.Signer.JsonRPC (via personal_sign).
• Add top-level Ethers.sign_message/2 and sign_message!/2, plus comprehensive tests and changelog entry.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/ethers/signer/local_test.exs	Adds Local signer sign_message/2 test vectors and JsonRPC parity check.
test/ethers/signer/json_rpc_test.exs	Adds JsonRPC signer personal_sign coverage and verification assertions.
test/ethers/personal_message_test.exs	New end-to-end unit tests for hashing, recovery, verification, and Ethers.sign_message/*.
lib/ethers/signer/local.ex	Implements sign_message/2 for local secp256k1 signing over the EIP-191 digest.
lib/ethers/signer/json_rpc.ex	Implements sign_message/2 delegating to JSON-RPC personal_sign.
lib/ethers/signer.ex	Extends signer behaviour with optional sign_message/2 callback and docs.
lib/ethers/personal_message.ex	Adds EIP-191 personal message primitives (hash, recover, verify).
lib/ethers.ex	Adds top-level sign_message/2 and sign_message!/2 routing through configured signer.
CHANGELOG.md	Documents the new EIP-191 personal message support.

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