feat(zkp): audit-friendly private transaction receipts - #107

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v3, actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

formal_verification_tests failed; full property tests are a follow-up after the 36-error user-code triage.

error_code_tests failed; in scope of 36-error triage.

kyc_tests failed; in scope of 36-error triage.