Add Swedish Bankgiro number validation

[cedendahlkim]

Summary

Add stdnum.se.bankgiro module for validating Swedish Bankgiro numbers.

Bankgiro numbers are used in Sweden to identify bank giro accounts for invoice payments and transfers. They consist of 7 or 8 digits with a Luhn (mod-10) check digit — the same algorithm already used by stdnum.se.orgnr and stdnum.se.personnummer.

Changes

• New module: stdnum/se/bankgiro.py — implements validate(), is_valid(), compact(), format()
• New tests: tests/test_se_bankgiro.doctest — covers valid/invalid numbers, formatting, edge cases

Design

Follows the established patterns from stdnum.se.orgnr:

• Uses stdnum.luhn.validate() for the mod-10 check
• Accepts both 7-digit (NNN-NNNN) and 8-digit (NNNN-NNNN) formats
• The format() function hyphenates at position 3 or 4 depending on length

Testing

>>> from stdnum.se.bankgiro import validate, is_valid, format
>>> validate('9000001')
'9000001'
>>> validate('50501501')
'50501501'
>>> is_valid('9000002')
False
>>> format('9000001')
'900-0001'

References

• https://www.bankgirot.se/en/
• Built using the Swedish Bankgiro GitHub Action as reference implementation (part of the Swedish DevOps Toolkit)