Add Roslynator RCS1213 analyzer to detect unused members

[currantw]

Summary

Add Roslynator's RCS1213 (unused member) analyzer to detect dead code that the built-in .NET analyzers miss.

Motivation

The built-in IDE0051/IDE0052 analyzers only flag private unused members. Roslynator's RCS1213 covers broader visibility levels (internal, protected, public) and would help catch and remove dead code.

Proposed Approach

• Install roslynator.dotnet.cli as a dotnet tool (add to .config/dotnet-tools.json)
• Add a lint step that runs roslynator analyze --include RCS1213
• Alternatively, add Roslynator.Analyzers as a NuGet package reference for build-time analysis

Requires .NET 10 SDK (already available in CI).