Verify PostSharp Tools for Visual Studio load in Visual Studio 18.9 and 18.10

[gfraiteur]

The extensions under UserInterface/ must load in Visual Studio 2026 versions 18.9 and 18.10, and
must not fail on C# 15 syntax in the editor. The scope depends on how much of the tooling parses
source code itself.

Analysis: Documentation/DotNet11-CSharp15-Impact.md in postsharp/PostSharp.

— Claude for Gael

[gfraiteur]

Context on the Visual Studio release model, which adds scope to this issue.

Microsoft documents an annual Visual Studio release each November, aligned with the new major version
of .NET, quoting the release rhythm page: "beginning with Visual Studio 2026 we plan to deliver new
annual releases each November along with the new major version of .NET. These annual releases will be
in-place updates to the prior annual year's release, rather than side-by-side as with Visual Studio
2022 and earlier."

Three consequences for the extensions under UserInterface/:

1. A new annual release is due in November 2026, alongside .NET 11. Its name and major version number
   have not been announced, so it is not yet known whether it continues the 18.x line or moves to
   19.x. That distinction matters for the extension manifests and for
   Root.Validation.targets.
2. The annual release is an in-place update rather than a side-by-side installation. The upgrade path
   for an installed PostSharp extension differs from the Visual Studio 2022 model, where a new
   product version meant a new installation.
3. Visual Studio 2026 gets a Long-Term Servicing Channel release in November 2026, supported for a
   further year. Each annual release receives one year of feature updates and servicing plus one year
   of security servicing as an LTSC. PostSharp will therefore need to support the 18.x LTSC and the
   next annual release at the same time, through to late 2027.

Consider whether supporting the November 2026 annual release deserves its own issue once its version
number is announced.

Source: Visual Studio channels and release rhythm.

— Claude for Gael