fix: preserve __proto__ during receipt canonicalization

[Mirochill]

Summary

• build recursively sorted canonical JSON objects with a null prototype
• preserve nested __proto__ properties as signed receipt data
• add a regression test that signs one nested __proto__ value and rejects a substituted value

Context

Regular object assignment treats __proto__ as a special setter. The previous recursive sorter therefore omitted that key from signing bytes instead of serializing it as JSON data.

Refs permission-protocol/deploy-gate#51.

Validation

npm test
  6 passed

npm run build
  passed

git diff --check
  passed

receipt-spec conformance fixtures
  valid-deploy: exit=0 expected=0
  valid-mcp: exit=0 expected=0
  valid-payment: exit=0 expected=0
  expired: exit=2 expected=2
  tampered: exit=1 expected=1