Add lightweight C# 12 refactoring sample

[balajibashyam]

Summary

Adds a minimal .NET 10 console companion for the C# 12 Language Features: Primary Constructors, Collections & More tutorial.

What's included

• New sample: csharp-language/csharp-12-features/ — 9 files

  • TodoItem with class primary constructor, explicit properties, title normalization
  • TodoService with service primary constructor, collection expressions, spread elements, tuple-type alias
  • TodoFormatter with default lambda parameter
  • Deterministic console output with 5 exact expected lines
  • 7 unit tests covering properties, reflection isolation, seed/snapshot copying, spread immutability, tuple counts, lambda defaults, and active filtering

• Root README: New sample row in the table + csharp-language/ tree section

• GitHub Actions: New test-csharp-12-features job with LangVersion verification and console-output checks

Deliberately omitted (stays in the full tutorial)

• Inline arrays, stack allocation, unsafe code
• Benchmarks, allocation claims
• Minimal APIs, ASP.NET Core
• C# 11 list patterns, preview features

Versions

• .NET SDK: 10.0.302
• LangVersion: 12.0 (both projects, explicitly pinned)
• Application packages: none
• xunit.v3: 3.2.2

Validation

• dotnet restore — passed
• dotnet build --configuration Release — passed (0 errors, 0 warnings, TreatWarningsAsErrors)
• dotnet test --configuration Release — passed (7/7)
• dotnet msbuild -getProperty:LangVersion — 12.0 (app + test)
• Console output — matches expected exactly
• Security scan — clean