Pin .NET SDK to 10.0.204 via global.json

[erikdarlingdata]

Summary

Today's nightly build failed restoring `Installer/PerformanceMonitorInstaller.csproj` with:

```
error NU1004: The package reference Microsoft.NET.ILLink.Tasks
version has changed from [10.0.8, ) to [10.0.7, ).
The packages lock file is inconsistent with the project dependencies
so restore can't be run in locked mode.
```

`Microsoft.NET.ILLink.Tasks` is bundled with the .NET SDK, so its version is whatever SDK the GitHub runner image happens to ship. The lockfiles were generated against SDK 10.0.204 (runtime 10.0.8); the runner now ships SDK 10.0.107 (runtime 10.0.7), and `dotnet restore --locked-mode` correctly refused to silently rewrite the lock.

Fix

A `global.json` at the repo root pins the SDK:

```json
{
"sdk": {
"version": "10.0.204",
"rollForward": "latestFeature",
"allowPrerelease": false
}
}
```

`actions/setup-dotnet` reads `global.json` automatically and downloads SDK 10.0.204 (or higher within 10.0.x) on the runner if it isn't already present, regardless of what the runner image ships with. Local dev and CI now agree on the SDK, and the only way to bump it is to edit `global.json` and regenerate lockfiles in the same commit — failures will surface in PR CI, not as a 4am nightly surprise.

Test

```text
$ dotnet --version
10.0.204

$ dotnet restore Installer/PerformanceMonitorInstaller.csproj --locked-mode
Determining projects to restore...
Restored .../Installer/PerformanceMonitorInstaller.csproj (in 476 ms).
Restored .../Installer.Core/Installer.Core.csproj (in 476 ms).
```

What's deliberately not in this PR

• `nightly.yml` on `main` still has the pre-.NET-10 `Setup .NET 8.0` step (`dotnet-version: 8.0.x`). dev's copy was already updated to `10.0.x` during the .NET 10 upgrade, so the next `dev → main` release sweeps that up automatically. No separate fix needed here.
• Lockfiles are not regenerated. With `global.json` in place, the existing 10.0.8-pinned lockfiles are correct.

🤖 Generated with Claude Code