Add a bounded DBF/xBase data reader

[PrzemyslawKlys]

Adds DBAClientX.Dbf, a standalone, forward-only DbDataReader for DBF/xBase ingestion without a database driver or SQL-provider dependency.

• Reads the documented dBASE III, FoxPro 2 and Visual FoxPro profiles, including supported DBT/FPT memos, typed values, native nulls and schema metadata.
• Provides path and stream opens, asynchronous table reads, explicit stream ownership, strict decoding and configurable byte, record, field and memo limits. Unsupported layouts, encrypted tables and malformed data fail explicitly.
• Keeps indexes, backlinks and embedded payloads inert. This is a read-only codec; it does not write DBF files or interpret embedded objects.

OfficeIMO consumes this canonical codec through its thin OfficeIMO.Reader.Dbf adapter in OfficeIMO #3001. The downstream adapter consumes the published DBAClientX.Dbf 1.0.11 package.

Validation covers 28 focused cases on .NET 8 and .NET 10, independently produced and decoded fixtures, and an unsigned package for .NET Standard 2.0, .NET 8 and .NET 10. The existing Windows package compatibility lane now includes this project. Original dBASE/FoxPro application acceptance and NativeAOT remain unqualified.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.88%. Comparing base (7954c4e) to head (3b4f947).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage   55.84%   55.88%   +0.04%     
==========================================
  Files         447      447              
  Lines       31384    31389       +5     
  Branches     6262     6265       +3     
==========================================
+ Hits        17526    17542      +16     
+ Misses      11909    11900       -9     
+ Partials     1949     1947       -2     

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