chore(build): adopt central package management (Directory.Packages.props) for test-tooling versions #240

Motivation

PR #239 (NUnit 3 to 4.6.1 upgrade) surfaced this follow-up. All 9 test csprojs now duplicate the same package versions row-for-row:

• NUnit@4.6.1
• NUnit3TestAdapter@4.6.0
• Microsoft.NET.Test.Sdk@17.14.1
• coverlet.collector@6.0.4

The first drift will be silent, and #239 itself resulted from precisely this pattern (four csprojs had drifted onto NUnit 3.13.3 + adapter 4.3.1/4.5.0 while the other five sat on 3.14.0 + 4.6.0).

Proposal

Add Directory.Packages.props at the repo root with:

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup>
    <PackageVersion Include="NUnit" Version="4.6.1" />
    <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
    <PackageVersion Include="coverlet.collector" Version="6.0.4" />
    <!-- plus every other cross-cutting dependency in the tree -->
  </ItemGroup>
</Project>

Strip the Version= attribute from every <PackageReference> across the tree.

Scope

• All test csprojs (9)
• All library csprojs (packages currently duplicated across projects)
• All adapter / agent / module / example csprojs

Sequencing

Should land before the follow-up Stryker.NET adoption PR — Stryker will add yet another cross-cutting test-only package, and CPM saves that PR from another N-way sync.

Source

Improvement audit finding F-IMP-001 during PR #239 Ultrareview cycle 1 (2026-08-19). Related simplification / code-review agents also flagged the duplication as a maintenance risk.