breaking: drop netstandard2.0, seal ValidationHelper, migrate to TUnit, modernize build

[glennawatson]

Breaking Changes

• netstandard2.0 removed - aligning with the ReactiveUI upstream dependency which no longer targets netstandard2.0. Consumers on netstandard2.0 should migrate to a .NET Framework (net462/net472/net481) or .NET (net8.0/net9.0/net10.0) target.
• net481 added to .NET Framework targets (net462, net472, net481).
• ValidationHelper sealed - no meaningful subclass scenarios exist; simplifies dispose pattern.
• Version bumped to 7.0 for the breaking changes above.

Supported Target Frameworks

Category	Targets
.NET	net8.0, net9.0, net10.0
.NET Framework	net462, net472, net481
Windows	net8.0-windows10.0.19041.0, net9.0-windows10.0.19041.0, net10.0-windows10.0.19041.0
Android	net9.0-android, net10.0-android

Test Coverage

Metric	Coverage
Line	99.3%
Branch	98.3%
Method	100%

Remaining uncovered lines are in legitimately reachable but hard-to-trigger paths (view property setter exceptions, chained property expressions). Provably unreachable defensive code was removed rather than left as dead code.

Maintainer Notes

• Test framework migrated from xUnit to TUnit with Microsoft Testing Platform (MTP).
• Microsoft.Testing.Extensions.CodeCoverage pinned at 18.4.1 - newer versions have a regression that produces incorrect coverage data. Do not upgrade without verifying coverage output.
• SLNX solution format replaces the legacy .sln file (ReactiveUI.Validation.slnx). Samples now contained in the main slnx file and moved from /samples.
• Central Package Management adopted via Directory.Packages.props - all package versions are managed centrally.
• API approval tests use Verify.TUnit - run tests to regenerate .verified.txt files when the public API surface changes.
• Release workflow updated to use workflow_dispatch, eSigner code signing, and OIDC NuGet trusted publishing (aligned with Splat/ReactiveUI).

Other Changes

• Samples and tests relocated under src/ for unified build
• Verbose null-check blocks replaced with ArgumentExceptionHelper using CallerArgumentExpression
• Manual backing fields replaced with C# field keyword for auto-properties
• Internal classes sealed (SingleValidationText, ArrayValidationText, ReadOnlyDisposableCollection<T>)
• Removed provably unreachable defensive code in BasePropertyValidation
• Added Polyfill package for downlevel framework support
• Added codecov.yml configuration
• Added CLAUDE.md with build, test, and architecture reference
• Updated README badges and removed deprecated Xamarin.Android package references

[codecov]

Codecov Report

❌ Patch coverage is 98.61111% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.38%. Comparing base (e561ef1) to head (5804488).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...eUI.Validation/Helpers/ReactiveValidationObject.cs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #876       +/-   ##
===========================================
+ Coverage   71.65%   99.38%   +27.73%     
===========================================
  Files          21       21               
  Lines         709      650       -59     
  Branches      151      107       -44     
===========================================
+ Hits          508      646      +138     
+ Misses        119        0      -119     
+ Partials       82        4       -78     

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[github-actions]

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