test: add DANFSe v2 golden reference contracts

[vitormattos]

Closes #57.

Summary

• add a single named DANFSe v2 structural reference suite
• cover common taxable invoice, retained ISS, foreign taker, long taxation descriptions, IBS/CBS, legacy XML without IBS/CBS, homologation marker and PDF artifact generation
• document the stable rendering guarantees and the deliberate non-guarantee of byte-identical PDF output
• use synthetic/public-schema data only; no copyrighted screenshot fixtures

Baseline strategy

The golden contract is semantic/structural rather than pixel- or PDF-byte-based, avoiding false churn from QR SVG/PDF metadata while still forcing intentional review of meaningful layout/data changes.