docs: update README and CHANGELOG for v2.6.2 multi-targeting

- Badge: .NET 8.0 -> .NET 8.0 | 10.0
- Install snippet: version 2.6.0 -> 2.6.2
- Requirements: .NET 8.0 or later -> .NET 8.0 or .NET 10.0 (both LTS)
- CHANGELOG: add 2.6.2 entry covering multi-targeting, CPM, package validation, MSTest upgrade, CA2264 fixes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rbenzing

CHANGELOG.md

@@ -7,6 +7,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [2.6.2] - 2026-06-19
+
+### Added
+- **Multi-Framework Targeting**: Library now targets both `net8.0` and `net10.0` (both LTS). NuGet package ships a `lib/net8.0/` and `lib/net10.0/` asset, giving consumers first-class builds on either runtime.
+- **Centralized Package Management**: All NuGet dependency versions centralised in `Directory.Packages.props`; no inline `Version=` attributes in project files.
+- **NuGet Package Source Mapping**: `NuGet.config` restricts all package resolution to `nuget.org`, preventing dependency-confusion attacks.
+- **Package Validation**: `EnablePackageValidation=true` enforces identical public API surface across both TFMs on every `dotnet pack`.
+- **CI TFM Matrix**: GitHub Actions now runs a `tfm: [net8.0, net10.0]` matrix axis so each framework is independently built and tested on every push.
+
+### Changed
+- **MSTest upgraded**: Test project bumped from 2.2.10 to 3.11.1.
+- **Microsoft.Extensions.\* upgraded**: `DependencyInjection.Abstractions` and `Logging.Abstractions` upgraded from 9.0.4 to 10.0.0, picking up native `net10.0` TFMs.
+- **Microsoft.CodeAnalysis.NetAnalyzers upgraded**: 8.0.0 → 10.0.100 (SDK RTM build).
+
+### Fixed
+- **CA2264**: Removed five dead `ArgumentNullException.ThrowIfNull()` calls on `KeyPair readonly struct` parameters (`DeviceLinkingService`, `ProtocolAdapter`, `DoubleRatchetProtocol`, `SessionPersistenceManager`, `X3DHProtocol`). `ThrowIfNull` on a non-nullable value type is a no-op; struct fields are already validated in the constructor.
+- **Microsoft.TestPlatform removed from library project**: The test-runner package was incorrectly referenced as a production dependency of `LibEmiddle.csproj`; it has been removed.
+
 ## [2.6.0] - 2026-03-21
 
 ### Added

README.md

@@ -3,7 +3,7 @@
 ![Build Status](https://img.shields.io/badge/build-passing-brightgreen)
 ![Coverage](https://img.shields.io/badge/coverage-90%25-brightgreen)
 ![License](https://img.shields.io/badge/license-AGPL--3.0-blue)
-![.NET](https://img.shields.io/badge/.NET-8.0-purple)
+![.NET](https://img.shields.io/badge/.NET-8.0%20%7C%2010.0-purple)
 ![Version](https://img.shields.io/badge/version-2.6.2-blue)
 
 A comprehensive, production-ready end-to-end encryption library for .NET applications implementing modern cryptographic protocols with a focus on security, privacy, and usability. Now with advanced features including post-quantum cryptography preparation, WebRTC transport, message batching, and enterprise-grade monitoring capabilities.
@@ -489,7 +489,7 @@ client.KeyRotated += (sender, args) =>
 
 ### NuGet Package
 ```bash
-dotnet add package LibEmiddle --version 2.6.0
+dotnet add package LibEmiddle --version 2.6.2
 ```
 
 ### Package Manager Console
@@ -498,7 +498,7 @@ Install-Package LibEmiddle -Version 2.6.2
 ```
 
 ### Requirements
-- .NET 8.0 or later
+- .NET 8.0 or .NET 10.0 (both LTS)
 - Windows, Linux, or macOS
 - libsodium native library (included in package)