CultMath.Tests is on xunit.v3. Stryker's default vstest runner cannot
capture coverage against xunit.v3's test host and hangs retrying test
sessions indefinitely (observed: 2096 mutants queued, zero completed after
~9 minutes of retries). Stryker's Microsoft.Testing.Platform runner
(-t mtp, still preview) works correctly against xunit.v3 and completed the
same 1351-mutant run in ~4.5 minutes. Pin it in config so a plain
`dotnet stryker` works without a CLI flag.
Add docs/mutation-testing.md: how to run it, the --since form for a single
cut, the triage rules, a known coverage-capture caveat in the mtp runner
(confirmed by hand-mutation against static readonly fields), and the
packages still needing their own config.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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