Twitch Archive (.NET) - Initial Scaffold

This folder contains the initial .NET 10 scaffold for the Twitch Archive rewrite.

Quick commands (from `dotnet/`):

```bash
dotnet build ./src/TwitchArchive.Core
dotnet test ./src/TwitchArchive.Tests
```

This initial commit includes a pure `RecoveryPolicy` implementation and unit tests.

Next steps: add services, process runner, Blazor Server app.