SystemClock

An attempt at an async/thread safe implementation of a testable .NET clock
Based on code from [Brad Wilson] (https://github.com/CenturyLinkCloud/NSubstitute/blob/f7eee68840eace09b757c500fadd47839222d22d/Source/NSubstitute/Core/RobustThreadLocal_Net45.cs)

CI / NuGet publishing

• GitHub Actions workflow (.github/workflows/ci.yml) runs build and tests on every push to main.
• Manual runs (workflow_dispatch) can optionally publish to NuGet by setting publish=true and providing package_version; otherwise the workflow only builds/tests.