Add Sign-In with Ethereum (EIP-4361) support

[alisinabh]

Third PR of the v0.7 "Auth" tier. Stacked on #266 (base is feat/universal-sig-verification; retarget after #266 merges).

What

• New Ethers.Siwe + Ethers.Siwe.Message implementing EIP-4361:
  • new/1 / new!/1 (field validation, EIP-55 address normalization, nonce rules), generate_nonce/0 (crypto-random, ≥8 alphanumeric)
  • to_message/1 (exact EIP-4361 rendering + String.Chars), strict parse/1 (byte-for-byte round-trip)
  • validate/2 — stateless checks with injectable time:: domain/scheme/nonce/address binding, expiration (exclusive) / not-before (inclusive), version
  • verify/3 — the one-call backend flow: parse → validate → signature check via Ethers.Signature.verify_message/4, so smart-wallet (ERC-1271/6492) sign-ins work out of the box; EOA sign-ins verify with zero RPC calls. {:ok, false} maps to {:error, :invalid_signature}.
• New guide guides/siwe.md with the Phoenix controller recipe (nonce in session → verify endpoint).

Design notes

• Timestamps are stored as RFC 3339 strings (not DateTime) so non-UTC offsets and sub-second precision round-trip exactly, matching spruceid's JS/Rust libraries; new/1 accepts DateTime and validate/2 parses on demand.
• Includes the optional scheme field from the current EIP-4361 ABNF.

Testing

• TDD; 100% coverage on both new modules.
• Parsed against the official spruceid reference vectors: all positive vectors parse, field-match, and round-trip byte-for-byte; all negative message/object vectors rejected.
• verify/3 covered end-to-end: EOA happy path under a raising RPC stub (proving no network use), wrong signer/message, every validation error short-circuiting before the signature check, and a deployed ERC-1271 wallet on anvil accepting its owner's signature.
• Full suite green (474 tests); mix format, mix credo --strict, mix dialyzer clean.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds first-class Sign-In with Ethereum (SIWE, EIP-4361) support to the Ethers library, including message construction/parsing/validation and an end-to-end verification helper that leverages the universal signature verification stack (EOA + ERC-1271/6492). It also introduces a dedicated guide and comprehensive fixtures/tests for reference vectors and verification flows.

Changes:

• Add Ethers.Siwe and Ethers.Siwe.Message implementing EIP-4361 creation, rendering, strict parsing, stateless validation, and signature verification.
• Add SIWE documentation guide and wire it into ExDoc.
• Add reference fixtures and ExUnit coverage for parsing/validation and EOA + ERC-1271 verification paths.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
lib/ethers/siwe.ex	Core SIWE implementation: build/parse/render/validate/verify.
lib/ethers/siwe/message.ex	Defines the SIWE message struct, types, and String.Chars impl.
guides/siwe.md	Phoenix integration guide and security notes for SIWE usage.
mix.exs	Adds the SIWE guide/modules to generated docs navigation/groups.
CHANGELOG.md	Documents new SIWE feature in the changelog.
test/ethers/siwe_test.exs	Unit tests for message lifecycle: new/render/parse/validate and vectors.
test/ethers/siwe_verify_test.exs	End-to-end verify/3 tests for EOA and ERC-1271 paths.
test/support/fixtures/siwe/parsing_positive.json	Official/reference positive parsing vectors for round-trip tests.
test/support/fixtures/siwe/parsing_negative.json	Reference negative raw-message vectors for strict parsing tests.
test/support/fixtures/siwe/parsing_negative_objects.json	Reference negative object vectors for new/1 validation tests.

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