docs: standardize README following org template

[phmatray]

Summary

• Added The Problem / The Solution sections explaining the pain of testing with System.IO and how VFS solves it
• Added Tech Stack table (C# 13, .NET 9/10, Blazor Server, Octokit, Spectre.Console, xUnit, Nuke)
• Added Architecture ASCII diagram showing the core abstractions (IVirtualFileSystem, VFS, node tree, undo/redo, event system, GitHub extension)
• Added Project Structure section with accurate directory tree from the actual repo
• Extracted planned features into a dedicated Roadmap section
• Added Stats section with RepoBeats placeholder (needs hash registration)
• Added branding footer with Atypical Consulting attribution and contributors image
• Reorganized Table of Contents to follow the standardized template order
• Preserved all existing content: badges, NuGet links, code examples, demo docs, contributing, license, acknowledgments

Test plan

• Verify all badge URLs resolve correctly
• Verify NuGet package link works
• Verify Table of Contents anchor links work
• Review architecture diagram for accuracy
• Confirm project structure matches actual repo layout