chore: add conformance evidence tooling and release gates

[greyllmmoder]

Summary

• add scripts/conformance_report.py to generate audit-ready conformance evidence (markdown + JSON)
• add docs/ISO_IEC_24778_TRACEABILITY.md with implementation-to-test traceability mapping
• add CI conformance gate and artifact upload in .github/workflows/ci.yml
• add .github/workflows/release-gate.yml to enforce quality + conformance + build checks for release events
• add smoke test for conformance script
• update README/checklist/manifest/gitignore to include the new conformance workflow artifacts

Testing

• python3 -m pytest -q
• python3 -m ruff check .
• python3 -m mypy --strict aztec_py
• python3 -m build
• python3 scripts/conformance_report.py --report conformance_report.md --json conformance_report.json --matrix-report compat_matrix_report.md

Notes / Risks

• no encoder algorithm behavior changes
• decode checks remain skip-safe without Java/ZXing unless --strict-decode is used