Add lightweight EF Core relationship loading sample

[balajibashyam]

Summary

Adds a minimal .NET 10 companion sample for the EF Core 8 Fundamentals: Modern Data Access with .NET 8 tutorial.

What's included

• New sample: ef-core/modern-data-access-dotnet/ — 11 files

  • Required one-to-many (Category → TodoItem) and many-to-many (TodoItem ↔ Tag) modeling
  • Intentional N+1 baseline with DbCommandInterceptor SELECT counting
  • Eager Include loading categories in one query
  • Filtered Include loading only urgent tags
  • AsSplitQuery loading the complete graph in three queries
  • Explicit reference loading (conditionally loading Category)
  • AsNoTracking on read-only eager-loading paths
  • 8 deterministic xUnit v3 tests

• Root README: New sample row + both EF Core companions in the tree

• GitHub Actions: New test-ef-core-relationships job; path filters updated

Deliberately omitted (stays in the full tutorial or in the existing advanced EF Core companion)

Web API CRUD, migrations, owned/complex types, JSON, compiled queries, ExecuteUpdate/Delete, concurrency, transactions, retries, SQL Server, PostgreSQL, lazy-loading proxies, raw SQL, Testcontainers, benchmarks

Versions

• .NET SDK: 10.0.302
• Microsoft.EntityFrameworkCore.Sqlite: 10.0.11

Validation

• dotnet restore — passed
• dotnet build --configuration Release — passed (0 errors, 0 warnings)
• dotnet test --configuration Release — passed (8/8)
• Exact deterministic console output verified
• Security scan — clean
• Package baseline — 1 direct package
• No SQLite database file generated