chore(extensions): migrate to @signinwithethereum/siwe

[caveman-eth]

Migrates @x402/extensions from the old Spruce siwe package (v2.3.2) to @signinwithethereum/siwe (Ethereum Identity Foundation, v4.1.0).

The Spruce siwe package is no longer actively maintained.

Stewardship of the SIWE standard has moved to the Ethereum Identity Foundation (GitHub). @signinwithethereum/siwe is the official successor TypeScript implementation. The new package:

• Supports viem natively as a peer dependency (alongside optional ethers)
• Has first-class EIP-1271 and EIP-6492 smart wallet verification
• Maintains the same SiweMessage API — constructor and prepareMessage() are identical

Changes:

File	Change
typescript/packages/extensions/package.json	"siwe": "^2.3.2" → "@signinwithethereum/siwe": "^4.1.0"
typescript/packages/extensions/src/sign-in-with-x/evm.ts	Updated import path
typescript/.changeset/migrate-siwe-to-eif.md	Changeset fragment

The legacy example at examples/typescript/legacy/fullstack/auth_based_pricing/ also depends on siwe directly — intentionally left untouched as it's in the legacy directory.

Tests

All 384 extension tests pass, including all 79 sign-in-with-x tests. No behavioral changes.

pnpm --filter @x402/extensions test

 ✓ test/eip2612-gas-sponsoring.test.ts (9 tests)
 ✓ test/facilitatorClient.test.ts (10 tests)
 ✓ test/erc20-approval-gas-sponsoring.test.ts (15 tests)
 ✓ test/payment-identifier.test.ts (77 tests)
 ✓ test/bazaar.test.ts (82 tests)
 ✓ test/offer-receipt.test.ts (112 tests)
 ✓ test/sign-in-with-x.test.ts (79 tests)

 Test Files  7 passed (7)
      Tests  384 passed (384)

Format and lint pass clean:

pnpm format  # All matched files use Prettier code style!
pnpm lint    # 0 errors

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)