Improve import UX and adopt EPPlus 8.6.1

[eraydin]

Summary

• upgrade EPPlus from 8.5.3 to 8.6.1 and prepare package version 3.2.0
• add structured Read<T>() results for mapping, casting, and validation errors
• add safe worksheet/table lookup helpers and named worksheet read APIs
• add clearer OnRow and export naming conveniences
• enumerate export sources only once
• document the new APIs

Why

Workbook imports currently require callers to choose between throwing or skipping errors, and common worksheet/table lookups rely on implicit null behavior. This change adds explicit, additive workflows while preserving all existing signatures, defaults, exception behavior, and default-literal call sites.

EPPlus 8.6.1 also brings the project onto the latest supported release and includes formula and workbook handling improvements.

Developer impact

Existing code continues to work unchanged. Consumers can opt into Read<T>(), TryGetWorksheet, TryGetTable, ToListFromWorksheet, AsEnumerableFromWorksheet, OnRow, and named XLSX output.

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