docs: add Slider partition validation guide

[0xC000005]

Summary

• add examples/SliderPartitionValidation, a runnable 8D Slider example that compares a correct grouped partition against an intentionally weak singleton partition
• expand Slider docs with a mental model, partition-selection workflow, and held-out validation snippet
• list the new example in docs, examples README, solution, and CI validation
• run VSTest with parallelization disabled to avoid the existing Console.Out capture race in verbose-output tests

Fixes #185 for the Slider/example portion.

Validation

• dotnet restore -p:NuGetAudit=false
• dotnet format --verify-no-changes --verbosity minimal --no-restore
• git diff --check
• dotnet build --configuration Release --verbosity minimal --tl:off -p:NuGetAudit=false
• dotnet run --configuration Release --no-build --project examples/QuickStart/QuickStart.csproj
• dotnet run --configuration Release --no-build --project examples/SliderPartitionValidation/SliderPartitionValidation.csproj
• dotnet run --configuration Release --no-build --project examples/TensorTrainHighDim/TensorTrainHighDim.csproj
• dotnet test tests/ChebyshevSharp.Tests/ChebyshevSharp.Tests.csproj --configuration Release --no-build --verbosity minimal --collect:"XPlat Code Coverage" -- RunConfiguration.DisableParallelization=true
• docfx docs/docfx.json (succeeded with existing local NU1900 warnings from the sandboxed NuGet vulnerability endpoint)

Notes

• A normal parallel local dotnet test run exposed an existing Console.Out capture race. The non-parallel VSTest setting passed all 1,649 tests and is now used in CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[0xC000005]

Superseded by #188, which combines the Slider and Tensor Train documentation updates with the 0.13.3 version bump and changelog entry.