release: v3.2.0 - improve import UX and adopt EPPlus 8.6.1

[eraydin]

Summary

• merge the completed v3.2.0 work from dev into main
• upgrade EPPlus to 8.6.1
• add structured import results and safer workbook lookup APIs
• add named worksheet read and export conveniences
• preserve backward compatibility and organize tests within the existing suites

Validation

• Release build succeeds with 0 warnings
• 150 tests pass
• NuGet package 3.2.0 packs successfully
• produced package depends on EPPlus 8.6.1
• NuGet vulnerability scan reports no vulnerable dependencies
• git diff --check passes