feat: Add NBIB parser and serializer

[chmodas]

Parses PubMed/MEDLINE NBIB files into Record structs and serializes them back. Handles tag lines, continuation lines, FAU/AU author preference, DOI extraction from LID/AID, date parsing, and format-specific extras (PMID, PMC).

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 99.05213% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.47%. Comparing base (ed16101) to head (0121fcc).

Files with missing lines	Patch %	Lines
src/nbib.rs	99.05%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           trunk       #2       +/-   ##
==========================================
+ Coverage   0.00%   92.47%   +92.47%     
==========================================
  Files          2        3        +1     
  Lines         15      226      +211     
==========================================
+ Hits           0      209      +209     
- Misses        15       17        +2     

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