Keep the netstandard2.0 public API available on .NET

[tmilnthorp]

Part of #1735.

Summary

The netstandard2.0 assembly exposed public members that were missing from the .NET assemblies. A netstandard2.0 library calling them failed with MissingMethodException when used from a .NET app. This PR makes those members available on every target framework and turns on package validation so CI catches future differences.

Changes

• Interfaces: IQuantityInfo and IQuantityInstanceInfo<TQuantity> are now public on all target frameworks. They used to be internal on .NET.
• IQuantityOfType<TQuantity>.QuantityInfo: now exists on all target frameworks.
  • On .NET it's [Obsolete] with the same message as IQuantity.QuantityInfo, pointing to TSelf.Info.
  • IQuantity<TSelf, TUnitType> implements it once as a default interface member, so generated quantities don't change.
• Temperature tolerance Equals: the two AffineQuantityExtensions overloads were netstandard2.0-only and now exist everywhere. On .NET, calls with Temperature arguments resolve to these non-generic overloads instead of the generic ones. Both compute the same comparison.
• Package validation: <EnablePackageValidation>true</EnablePackageValidation> in UnitsNet.csproj.
  • CompatibilitySuppressions.xml lists the intended exceptions, with a comment explaining them: the static abstract members (Info, Create, From, Zero) and the IAdditiveIdentity constraints. These require .NET 7+ and can't exist on netstandard2.0.
  • The PR workflow already packs, so validation runs on every PR.

Not in this PR

ILogarithmicQuantity<TSelf>.LogarithmicScalingFactor is still an instance property on netstandard2.0 and a static abstract property on .NET. Package validation doesn't flag it, because it matches members by name. Fixing it means breaking either the netstandard2.0 or the .NET API, so it needs a decision first. #1735 lists the options.

Testing

• Cross-framework repro (netstandard2.0 library used from a net10.0 app):
  • IQuantityOfType<T>.QuantityInfo and Temperature.Equals(other, tolerance): MissingMethodException on master, work with this PR.
  • LogarithmicScalingFactor: still fails, as described above.
• Package validation: a fresh dotnet pack passes. Before this change it reported 14 errors.
• New test: QuantityInfo_IQuantityOfTypeReference_ReturnsQuantityInfo.
• Full UnitsNet.Tests on net10.0: 52,559 passed, 2 failed. The failures are FeetInchesTests.TryParseFeetInches with de-CH, which fail the same way on master (see Fix feet/inches parsing with apostrophe grouping #1724).

🤖 Generated with Claude Code

https://claude.ai/code/session_012XKhDsyHDc5BHrmibxScqG

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98%. Comparing base (3bf1ef4) to head (b05e9c7).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1736   +/-   ##
======================================
  Coverage      98%     98%           
======================================
  Files         515     515           
  Lines       24092   24093    +1     
======================================
+ Hits        23692   23693    +1     
  Misses        400     400           

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net48	97% <ø> (ø)

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