[ScottishTapWater]

Summary

• Target netstandard2.0 for the runtime and Roslyn packages, using PolySharp for compiler features unavailable in the target framework.
• Replace Rational generic math interfaces with conversions and operators for common numeric types, retaining exact rational-to-floating-point rounding.
• Update packaged-consumer coverage and package layout checks for the new target.

Verification

• Release solution build passed.
• Runtime tests: 311 passed; Roslyn tests: 136 passed; packaged integration test: 1 passed.
• Package layout check passed on the generated nupkg.
• dotnet format --verify-no-changes --no-restore passed for changed C# files.
• Independent C# code review found no material issues.

The repository pins SDK 10.0.402, which is not installed locally. Verification used the installed 10.0.113 SDK with a temporary local pin, then restored global.json before commit.