[Bug]: PolySharp is credited in the README but never referenced by the templates

### Description

The root `README.md` credits [PolySharp](https://github.com/Sergio0694/PolySharp) in the *Credits* section:

> Polysharp - Generated, source-only polyfills for C# language features by Sergio Pedri

But no template references the package. There is no `PackageReference` to `PolySharp` anywhere under `templates/`.

This matters because the templates set `14.0` and multi-target down to `netstandard2.0` and `net47`. That is exactly the situation PolySharp exists to fix: modern C# language features such as records, `init` setters, required members and nullable attributes need types that the older frameworks do not ship.

### Expected behavior

Either PolySharp is referenced by the generated library project (so the newer language features actually work on the older target frameworks), or it is removed from the credits.

### Actual behavior

It is credited but not wired up, so users hit confusing compiler errors on the older target frameworks and have to discover the fix themselves.

### Other information

My preference would be to add it for real, since it is a `PrivateAssets="all"` development dependency and adds nothing to the shipped package.

Start in templates/ by locating the generated library project files and checking their existing PackageReference entries and target frameworks. Compare those references with the PolySharp credit in README.md. Done means the generated projects consistently wire PolySharp as intended for the older targets, or the unused credit is removed.

Written by the indexing model from the issue text.