[repo-assist] chore: introduce Central Package Management (CPM)

[github-actions]

🤖 This is an automated PR from Repo Assist.

What

Introduces [Central Package Management (CPM)]((learn.microsoft.com/redacted) by adding a Directory.Packages.props file at the solution root.

Changes

• Directory.Packages.props (new): declares ManagePackageVersionsCentrally=true and lists all 8 NuGet package versions used across the solution.
• tests/FixedPointNano.Tests/FixedPointNano.Tests.csproj: removed Version attributes from all PackageReference items.
• benchmarks/FixedPointNano.Benchmarks/FixedPointNano.Benchmarks.csproj: removed Version attribute from BenchmarkDotNet reference.

Why

Package versions were previously scattered across two .csproj files. With CPM, all version pins live in one file — future upgrades (e.g. bumping NUnit or the Test SDK) require editing only Directory.Packages.props rather than finding every project that references the package.

No functional changes; this is purely a build/maintenance improvement.

Test Status

✅ Build succeeded (dotnet build -c Release), 1437 tests pass (dotnet test -c Release), 0 warnings, 0 errors.

Targets: develop

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Warning

Protected Files

This was originally intended as a pull request, but the patch modifies protected files. These files may affect project dependencies, CI/CD pipelines, or agent behaviour. Please review the changes carefully before creating the pull request.

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Protected files

• Directory.Packages.props

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