Revisit vendored JetBrains.Annotations — migrate to System.Diagnostics.CodeAnalysis

[craigsmitham]

Summary

We have a vendored 1273-line src/Shared/JetBrainsAnnotations.cs containing dozens of ASP.NET MVC, Razor, XAML, and WPF attributes that this codebase will never use. Actual usage is narrow — we should evaluate migrating to built-in .NET equivalents and removing the vendored file.

Current usage

Attribute	Used in	Modern .NET replacement
[NotNull]	Check.cs	NRT (already enabled) handles this
[ContractAnnotation("=> halt")]	Check.cs, one extension method	[DoesNotReturn] from System.Diagnostics.CodeAnalysis
[InvokerParameterName]	Check.cs	[CallerArgumentExpression]
[NoEnumeration]	Check.cs	No direct replacement (can drop — generic constraint achieves the same)
[UsedImplicitly]	~20 syntax extension classes	SuppressMessage or remove if public API
[PublicAPI]	Generated accessors	Public members don't need unused-member suppression

What to do

1. Audit all usages of JetBrains.Annotations attributes across the codebase
2. Migrate each usage to the System.Diagnostics.CodeAnalysis equivalent where one exists
3. For [UsedImplicitly] / [PublicAPI], evaluate whether suppression is still needed
4. Delete src/Shared/JetBrainsAnnotations.cs
5. Remove the JetBrains.Annotations mandatory import from GraphZen.sln.DotSettings and the global using from src/Directory.Build.props

Notes

• We already have System.Diagnostics.CodeAnalysis as a global using
• This would eliminate 1200+ lines of dead code
• The 8 files with explicit using JetBrains.Annotations statements would also need cleanup