Add migration-guide for 4.0.0 (#604)

* Add migration-guide for 4.0.0

* Remove .NET 4.6.X compatibility documentation

Author: abergs

CLAUDE.md

@@ -0,0 +1,120 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This is the FIDO2 .NET Library (WebAuthn) - a working implementation library for FIDO2 and WebAuthn using .NET. It provides a developer-friendly and well-tested .NET FIDO2 Server/WebAuthn relying party library for validation of registration (attestation) and authentication (assertion) of FIDO2/WebAuthn credentials.
+
+## Architecture
+
+The solution is organized into several key projects:
+
+### Core Library Projects
+
+- **Src/Fido2.Models** - Core data models and DTOs for FIDO2/WebAuthn, shared across all projects
+- **Src/Fido2** - Main FIDO2 library with attestation/assertion verification logic, cryptographic operations, and metadata service integration
+- **Src/Fido2.AspNet** - ASP.NET Core integration helpers and extensions
+- **Src/Fido2.Ctap2** - CTAP2 protocol implementation for FIDO2 authenticators
+- **Src/Fido2.BlazorWebAssembly** - Blazor WebAssembly-specific components and helpers
+- **Src/Fido2.Development** - Development and testing utilities
+
+### Demo/Example Projects
+
+- **Demo** - ASP.NET Core demo application showing FIDO2 registration and authentication flows
+- **BlazorWasmDemo** - Blazor WebAssembly demo application (Client/Server projects)
+
+### Test Projects
+
+- **Tests/Fido2.Tests** - Main unit test suite using xUnit
+- **Tests/Fido2.Ctap2.Tests** - Tests for CTAP2 functionality
+
+## Development Commands
+
+### Building
+
+```bash
+# Build entire solution
+dotnet build fido2-net-lib.sln --configuration Release
+
+# Build specific project
+dotnet build Demo/Demo.csproj --configuration Release
+```
+
+### Testing
+
+```bash
+# Run all tests
+dotnet test Tests/Fido2.Tests/Fido2.Tests.csproj
+
+# Run tests with coverage
+dotnet test Tests/Fido2.Tests/Fido2.Tests.csproj --collect:"XPlat Code Coverage"
+```
+
+### Code Formatting
+
+```bash
+# Check formatting
+dotnet format --verify-no-changes --no-restore
+
+# Apply formatting
+dotnet format
+```
+
+### Running Demo Applications
+
+```bash
+# Run ASP.NET Core demo (requires HTTPS, expected at https://localhost:44329)
+dotnet run --project Demo/Demo.csproj
+
+# Run Blazor WebAssembly demo
+dotnet run --project BlazorWasmDemo/Server/BlazorWasmDemo.Server.csproj
+```
+
+## Configuration
+
+### Key Configuration Files
+
+- **Directory.Build.props** - Root build configuration with package metadata and global settings
+- **Src/Directory.Build.props** - Source-specific build settings
+- **azure-pipelines.yml** - CI/CD pipeline configuration
+
+### Important Settings
+
+- Target Framework: .NET 8.0 (configured via `SupportedTargetFrameworks`)
+- Language Version: C# 12
+- Nullable reference types enabled
+- ImplicitUsings enabled globally
+- TreatWarningsAsErrors enabled
+
+## Code Style Guidelines
+
+- Use 4 spaces for indentation (no tabs)
+- Use `_camelCase` for private fields
+- Always specify member visibility explicitly
+- Use `var` keyword when type is obvious
+- Use primitive type keywords (`int` vs `Int32`)
+- Document public APIs with XmlDoc comments
+- Avoid `this.` unless necessary
+- Use expression-bodied members when appropriate for readability
+
+## Key Dependencies
+
+- **NSec.Cryptography** - Cryptographic operations
+- **System.Formats.Cbor** - CBOR encoding/decoding
+- **Microsoft.IdentityModel.JsonWebTokens** - JWT token handling
+- **Microsoft.Extensions.Http** - HTTP client factory
+
+## Testing Framework
+
+- **xUnit** - Primary test framework
+- **Moq** - Mocking framework
+- **coverlet.collector** - Code coverage collection
+
+## Important Notes
+
+- The project passes 100% of FIDO Alliance conformance tests
+- Supports all current attestation formats: packed, tpm, android-key, android-safetynet, fido-u2f, apple, apple-appattest, and none
+- Includes FIDO Metadata Service V3 validation support
+- Cross-platform compatible (Windows, macOS, Linux)
+- Root namespace is `Fido2NetLib` for all projects