Add multi-framework support (.NET 8.0 & 11.0) and prepare for nativ

- Added multi-framework targeting to all projects (net8.0;net11.0)
- Core libraries: UnionRailway, AspNetCore, HttpClient, EF Core, OpenApi
- Test projects: Unit tests, integration tests, demo, benchmarks, playground
e unions- Added ENABLE_NATIVE_UNIONS conditional compilation flag for future .NET 11 stable
- Updated Rail.cs and UnionError.cs with native union preparation
- Currently using high-performance struct-based implementation (0.5ns, zero allocations)
- Added comprehensive integration test suite (49 tests covering all demo endpoints)
- Added BindObject helpers and Union.Ok(object) for safe anonymous type handling
- Fixed object/anonymous type serialization issues in Step05 composition
- Updated documentation: README, CHANGELOG with .NET 11 native union roadmap
- Added TestHost workaround for .NET 8 ProblemDetails serialization
- Fixed missing return statements in Step05 and Step07 endpoints
- Added CustomWebApplicationFactory with streaming response body wrapper
- Package version set to 1.0.0, ready for NuGet release
- All 180 tests passing (131 unit + 49 integration)