Feature/escrow currency metadata

[teeschima]

CLOSES #47 Add Invoice Currency Metadata Validation

✅ Verification & CI

All local checks passed:

Step | Command | Result -- | -- | -- Format | cargo fmt --all -- --check | ✅ No formatting errors Build | cargo build | ✅ Builds successfully Test | cargo test --features testutils | ✅ All tests pass Coverage | cargo llvm-cov --features testutils --fail-under-lines 95 --summary-only | ✅ ≥ 95% coverage

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💡 Notes

• Validation reduces invalid input / economic attack risks.
• Supports consistent downstream settlement and reporting.
• Future improvements: integrate with multi-escrow workflows and enforce role-based access for currency updates.