docs: update README with badges and development tool links

[2228293026]

Motivation

• Improve onboarding for developers by surfacing language/IDE badges and explicit links for C#/.NET/Visual Studio and clarifying the local build steps in README.md.

Description

• Added C#/Visual Studio/Downloads shields to README.md, introduced a 开发工具下载（编程语言 / 编译器） section with links to C# (https://learn.microsoft.com/dotnet/csharp/), .NET Framework (https://dotnet.microsoft.com/download/dotnet-framework), and Visual Studio (https://visualstudio.microsoft.com/zh-hans/downloads/), and renumbered the build instructions so the original 4.2 local build steps are now under 4.3 following the tooling guidance.

Testing

• No automated tests were run because this is a documentation-only change to README.md.

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Codex Task