Register ISO 7064 mod 97-10 checksum for BA, not BT (#264)

[CedricConday]

Closes #264

In schwifty/checksum/iso7064_mod97_10.py the ISO 7064 mod 97-10 national checksum algorithm is registered for BT:

# Bosnia and Herzegovina (BT)
...
@checksum.register("BT", "ME", "MK", "PT", "RS", "SI", "TL")

But the country code for Bosnia and Herzegovina is BA — BT is Bhutan, which is not part of the IBAN registry. So the algorithm was attached to a non-IBAN country, and BA IBANs never had their national checksum validated:

>>> from schwifty import IBAN
# valid IBAN-level checksum, invalid national check digits
>>> IBAN("BA551290079401028400", validate_bban=True)   # accepted (bug)

Fix

Register the algorithm for BA instead of BT.

Verified that Bosnia genuinely uses this algorithm: real BA IBANs (e.g. BA391290079401028494, BA391011606058553319) pass validate_bban=True, while a BA IBAN with corrupted national check digits is now correctly rejected with InvalidBBANChecksum. The other countries in the list (ME/MK/PT/RS/SI/TL) are unchanged.

Tests

Added two assertions to test_validate_national_checksum (valid BA passes, corrupted BA raises). They fail on main and pass with the fix. Full suite stays green (395 tests). Added a CHANGELOG entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.97%. Comparing base (5b1b0a1) to head (291c644).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
- Coverage   94.30%   93.97%   -0.34%     
==========================================
  Files          22       22              
  Lines        1211     1211              
  Branches       93       93              
==========================================
- Hits         1142     1138       -4     
- Misses         55       58       +3     
- Partials       14       15       +1     

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