Subtype markers are moved from .csproj to .csproj.user

### Environment

VS 2026

### .NET version

.net 10

### Did this work in a previous version of Visual Studio and/or previous .NET release?

i don't think this is a regression

### Issue description

existing
`
Form

`
entries in the .csproj files are moved to the .csproj.user file arbitrarily (it happened to a coworker, and i just saw it in the changelog)
is this specified behaviour or a bug?

my goal would be to set these SubType-Markers in a Directory.Build.Props file for all *Form.cs files and all *UserControl.cs files in our codebase - will that work?

### Steps to reproduce

I manually moved the values for a couple of forms into the .csproj because i moved the project in source control and the .csproj.user was stuck - and a day later, my change was reversed :-)

### Diagnostics

```text

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