Extract logic and hardcoded values about compiler's TFM into a single source of truth

[T-Gro]

Fixes #19204

Architecture: Single Source of Truth

┌─────────────────────────────────────────────────────────────────────────────┐
│                     eng/TargetFrameworks.props                               │
│              FSharpNetCoreProductDefaultTargetFramework                      │
│                   (SINGLE SOURCE OF TRUTH)                                  │
│                                                                             │
│  Default: net10.0                                                           │
│  Source-build (DotNetBuildSourceOnly=true): $(NetCurrent)                    │
└─────────────────────────────────────────────────────────────────────────────┘
                                    │
        ┌───────────────────────────┼───────────────────────────┐
        ▼                           ▼                           ▼
┌───────────────────┐    ┌─────────────────────┐    ┌─────────────────────┐
│     SCRIPTS       │    │      MSBUILD        │    │    COMPILED CODE    │
│  (--getProperty)  │    │   (Import props)    │    │    (generated)      │
└───────────────────┘    └─────────────────────┘    └─────────────────────┘
        │                           │                           │
        ▼                           ▼                           ▼
┌───────────────────┐    ┌─────────────────────┐    ┌─────────────────────┐
│ dotnet msbuild    │    │ Directory.Build.props│   │ buildproperties.fs  │
│ TargetFrameworks  │    │ imports props →     │    │ (auto-generated)    │
│ .props            │    │ sets:               │    │                     │
│ --getProperty:    │    │ • $(FSharp...TFM)   │    │ fsProductTfm =      │
│ FSharpNetCore...  │    │ • $(FSharp...Major) │    │   "net10.0"         │
│                   │    │                     │    │ fsProductTfmMajor = │
│ build.sh forwards │    │         ▼           │    │   10                │
│ /p:DotNetBuild    │    │ All .fsproj, .props │    │                     │
│ SourceOnly for    │    │ .targets reference  │    │         ▼           │
│ source-build      │    │ $(FSharp...TFM)     │    │ ┌─────────────────┐ │
│                   │    │                     │    │ │  PRODUCT CODE   │ │
│ Used by:          │    │ Used by:            │    │ │ FxResolver.fs   │ │
│ • Build.ps1       │    │ • TargetFramework   │    │ │ CompilerLoc.fs  │ │
│ • build.sh        │    │ • Output paths      │    │ │ uses FSharp.    │ │
│ • build-utils.ps1 │    │ • UseLocalCompiler  │    │ │ BuildProperties │ │
│ • test-determ.ps1 │    │ • checkpackages     │    │ └─────────────────┘ │
│ • ilverify.ps1    │    │ • EndToEnd tests    │    │                     │
│ • regression.yml  │    │                     │    │ ┌─────────────────┐ │
│ • CMD scripts     │    │                     │    │ │   TEST CODE     │ │
│                   │    │                     │    │ │ TestFramework.fs│ │
│                   │    │                     │    │ │ uses FSharp.    │ │
│                   │    │                     │    │ │ BuildProperties │ │
│                   │    │                     │    │ └─────────────────┘ │
└───────────────────┘    └─────────────────────┘    └─────────────────────┘

What changed

• Created eng/TargetFrameworks.props as the single source of truth for the product TFM, including source-build override (DotNetBuildSourceOnly=true → $(NetCurrent))
• All MSBuild files import this props file instead of hardcoding TFM values
• All scripts use dotnet msbuild --getProperty:FSharpNetCoreProductDefaultTargetFramework instead of hardcoded values
• build.sh forwards /p:DotNetBuildSourceOnly to ensure correct TFM resolution in source-build/VMR scenarios
• Directory.Build.props simplified — the <Choose> block for source-build override is now handled by TargetFrameworks.props
• Test code uses the auto-generated FSharp.BuildProperties module instead of reading text files
• Removed eng/productTfm.txt (superseded by props file)

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/10.0.300.md	No current pull request URL (#19251) found, please consider adding it

[ViktorHofer]

This looks good. I'm surprised that there are so many places in scripts and even in YML that need the TFM. That's a bit unusual but is fine.

One important feedback: Whenever you call into dotnet msbuild, make sure that you pass global properties in as well. Otherwise conditions on DotNetBuildSourceOnly=true are wrong and the TFM would be wrong.

[ViktorHofer]

Remove / change this block now?

fsharp/Directory.Build.props

Lines 72 to 87 in 1870673

	<Choose>
	<!-- Once we move to OOP in VS, and major dependants of FCS will move to netcore (not netstandard),
	we should also support $(NetPrevious) for all releases.
	This will likely include FCS and FSharp.Core as well as shipped products.
	Right now, it only covers products we ship (FSC and FSI), not NuGet packages. -->
	<When Condition="'$(DotNetBuildSourceOnly)' == 'true'">
	<PropertyGroup>
	<FSharpNetCoreProductTargetFramework>$(NetCurrent)</FSharpNetCoreProductTargetFramework>
	</PropertyGroup>
	</When>
	<Otherwise>
	<PropertyGroup>
	<FSharpNetCoreProductTargetFramework>$(FSharpNetCoreProductDefaultTargetFramework)</FSharpNetCoreProductTargetFramework>
	</PropertyGroup>
	</Otherwise>
	</Choose>

[T-Gro]

@ViktorHofer : Had exactly the same in mind , done :)

[ViktorHofer]

Is NetCurrent available in that new props file? The value comes from the Arcade SDK.