feat: add netstandard2.0 target and flattening cache

[bmgbeokwere]

feat: add netstandard2.0 target and flattening cache

Add netstandard2.0 as a second target framework alongside net10.0, making the library consumable from older runtimes. Fixes all API incompatibilities introduced by the broader language and runtime surface of net10.0, and adds a memoisation cache for flattened property accessor resolution.

• Add IsExternalInit polyfill to support record struct on netstandard2.0
• Replace ArgumentNullException.ThrowIfNull (.NET 6+) with explicit null guards
• Replace index/range syntax (^, ..) with Length-1 and Substring() calls
• Replace Dictionary.TryAdd with ContainsKey + Add for netstandard2.0
• Use #if NET5_0_OR_GREATER to select the correct GetMethod overload per target
• Fix erroneous static modifier on MapWithDepthTracking
• Cache TryBuildFlattenedGetter results keyed on (destPropertyName, sourceType) to eliminate redundant reflection on repeated configuration or validation calls
• Add three new flattening cache tests covering hit, key collision, and null miss
• Mark all private Profile subclasses in tests as sealed

[codacy-production]

Codacy's Analysis Summary

0 new issue (≤ 0 issue)
0 new security issue
19 complexity
0 duplications
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