fix(canonicalize): preserve nested __proto__ keys in signing bytes (closes #51)

[akuraposo]

Summary

Fixes #51: the receipt canonicalizer rebuilt nested objects with a
plain {} target, so a nested signed JSON object containing an own
__proto__ key was reassigned through Object.prototype's setter
instead of creating an own property. The key was then silently
omitted from the canonical JSON covered by the Ed25519 signature.

As a result, a nested requestJson.__proto__ value could be
changed after signing without changing the canonical bytes, and the
modified receipt would still pass pp verify.

Fix

• Inner sortKeys target is now Object.create(null), with field
  copy via Object.defineProperty. A nested __proto__ key is
  treated as an own enumerable string-keyed property of the result.
• Outer canonical object in canonicalizeReceipt uses the same
  null-prototype + defineProperty pattern, so the defense applies
  at the top level too.

Validation

• npm test -- --run — all 6 tests pass (5 pre-existing + 1 new
  regression test that forges a requestJson with a nested own
  __proto__ key, mutates its value, and asserts the canonical
  bytes change)
• Confirmed the new test FAILS on the pre-fix code and PASSES on
  the patched code
• npm run build — clean
• git diff --check — clean

Bounty

Submitted for assessment under #36 as a distinct Ed25519
verification-flow flaw. Payout details can be provided privately
after validation.