Build errors - experimental\coroutine(37,1): error C2338: static assertion failed, error C2653: 'stdext': is not a class,

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### Microsoft PowerToys version

main

### Installation method

Other (please specify in "Steps to Reproduce")

### Area(s) with issue?

General

### Steps to reproduce

Follow instructions to clone repo, install prerequisites and run `.\tools\build\build.ps1`

### ✔️ Expected Behavior

Build passes

### ❌ Actual Behavior

Build fails with the following errors:

```text
33>C:\Program Files\Microsoft Visual Studio\18\Enterprise\VC\Tools\MSVC\14.51.36231\include\experimental\coroutine(37,1): error C2338: static assertion failed: 'error STL1011: The /await compiler option, , , and are deprecated by Microsoft and will be REMOVED SOON. They are superseded by the C++20 and C++23 headers. You can define _SILENCE_EXPERIMENTAL_COROUTINE_DEPRECATION_WARNINGS to suppress this error for now.' [D:\git\PowerToys\src\common\CalculatorEngineCommon\CalculatorEngineCommon.vcxproj]
91>D:\git\PowerToys\deps\spdlog\include\spdlog\fmt\bundled\format.h(359,43): error C2653: 'stdext': is not a class or namespace name [D:\git\PowerToys\src\common\UnitTests-CommonUtils\UnitTests-CommonUtils.vcxproj]
```

### Upload Bug Report ZIP-file

_No response_

### Additional Information

Windows 11 25H2 26200.8457
VS 2026 with winget prerequisites

.NET SDK:
Version: 10.0.300
Commit: caa81fa497
Workload version: 10.0.300-manifests.e0989437
MSBuild version: 18.6.3+caa81fa49

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26200
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.300\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.8
Architecture: x64
Commit: 94ea82652c

.NET SDKs installed:
6.0.428 [C:\Program Files\dotnet\sdk]
8.0.301 [C:\Program Files\dotnet\sdk]
8.0.421 [C:\Program Files\dotnet\sdk]
9.0.314 [C:\Program Files\dotnet\sdk]
10.0.204 [C:\Program Files\dotnet\sdk]
10.0.300 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.27 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.27 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.27 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

### Other Software

_No response_

Start by running .\tools\build\build.ps1 with the reported Visual Studio 2026 environment and inspect the build output for CalculatorEngineCommon.vcxproj and UnitTests-CommonUtils.vcxproj. Read the reported experimental\coroutine and deps\spdlog\include\spdlog\fmt\bundled\format.h locations to identify the compatibility problem. Done means the repository build completes without these errors.

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