feat(cnpj): add alphanumeric CNPJ support per SERPRO 2026 spec

[rafaelkamimura]

Summary

Add support for Brazil's upcoming alphanumeric CNPJ format (effective 2026) while maintaining full backward compatibility with existing numeric CNPJs.

Changes

• CNPJv validator updated with alphanumeric support:

  • 4 regex patterns (numeric/alphanumeric x formatted/unformatted)
  • validate() override for alphanumeric char→value conversion
  • SERPRO algorithm: ord(char)-48, weights 2-9 R→L, mod 11

• Test infrastructure added:

  • tests/alphanumeric_cnpj.txt with valid test data
  • alphanumeric_cnpj_list fixture in conftest.py
  • test_alphanumeric_cnpj test function

• Version bump to 0.5.0 and added contributor

Specification

Based on SERPRO's official specification:

• Format: 12 alphanumeric chars + 2 numeric check digits
• Character values: ord(char) - 48 (0-9→0-9, A-Z→17-42)
• Example: 12ABC34501DE → DV=35 → 12.ABC.345/01DE-35

Reference: SERPRO - CNPJ Alfanumerico

Testing

All 21 tests pass, including:

• 88 existing numeric CNPJs (backward compatibility)
• New alphanumeric CNPJs (formatted and unformatted)
• Invalid alphanumeric rejection

============================= test session starts ==============================
tests/test_brdocs.py::test_cnpj PASSED
tests/test_brdocs.py::test_alphanumeric_cnpj PASSED
============================== 21 passed in 0.26s ==============================

Checklist

• Backward compatible with numeric CNPJs
• Supports formatted (12.ABC.345/01DE-35) and unformatted (12ABC34501DE35)
• Case-insensitive (both abc and ABC work)
• All tests pass
• No new dependencies