[samtrion]

Problem

Stryker solution mode logs Picking net10.0 for project .../NetEvolve.Arguments.csproj, although the project targets net6.0 to net10.0 plus netstandard2.0 (and net472/48/481 on Windows). The legacy polyfill branches (#if !NET7_0_OR_GREATER, #if !NET8_0_OR_GREATER, ...) compile to nothing on net10.0, so e.g. ObjectDisposedExceptionPolyfills.cs and the legacy parts of ArgumentExceptionPolyfills.cs / ArgumentOutOfRangeExceptionPolyfills.cs report 0 mutants.

Root cause

Only the highest TFM per source project is mutated; the polyfills are empty there by design.

Change

• New stryker-config.net6.json: target-framework: net6.0, mutates only **/*Polyfills.cs, only the NetEvolve.Arguments.Tests.Unit test project, no dashboard reporter (so the default run's dashboard report is not overwritten).
• New polyfills job in mutation.yml calling the same shared workflow with that config. The existing net10.0 run is untouched.
• Expected warning: Stryker logs that it cannot find a net6.0 target for the netstandard2.0-only Analyser project and falls back to its own pick.

Before / After

metric	before	after
mutants in polyfill files (net10.0 run)	0 for ObjectDisposedExceptionPolyfills.cs, ArgumentNullExceptionPolyfills.cs, legacy branches elsewhere	pending
mutants in net6.0 polyfill run	n/a	pending

Risk / what this does not change

• Feasibility is the open question: the Stryker docker image is dotnet/sdk:10.0 only, so the net6.0 test host may not launch. The numbers above come from a dispatched run on this branch.
• net6.0 does not compile the !NET6_0_OR_GREATER code (ArgumentNullExceptionPolyfills.cs, Polyfills/*); that only exists on netstandard2.0/net4x and is not covered here.
• Adds one extra scheduled job (small, polyfills only).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.75%. Comparing base (b49a9f3) to head (9cb16af).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #852   +/-   ##
=======================================
  Coverage   90.75%   90.75%           
=======================================
  Files          39       39           
  Lines        2185     2185           
  Branches      381      381           
=======================================
  Hits         1983     1983           
  Misses        121      121           
  Partials       81       81           

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