feat: Add EndNote XML parser and serializer

[chmodas]

Event-based implementation using quick-xml Reader/Writer API. Strips presentational <style> tags before parsing, handles XML entity references (GeneralRef events), numeric character references, self-closing elements, and both and bare root structures. Stores label and rec-number in extras.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 99.04306% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.74%. Comparing base (bfc67f2) to head (a648f86).

Files with missing lines	Patch %	Lines
src/endnote_xml.rs	99.03%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##            trunk       #4      +/-   ##
==========================================
+ Coverage   95.33%   96.74%   +1.41%     
==========================================
  Files           5        6       +1     
  Lines         407      615     +208     
==========================================
+ Hits          388      595     +207     
- Misses         19       20       +1     

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