Let FSharpProjectSnapshot.FromOptions take reference stamps from the host

[xperiandri]

Description

FSharpProjectSnapshot.FromOptions stats every -r: reference (FileSystem.GetLastWriteTimeShim) each time a snapshot is built from options. Under the transparent compiler in Visual Studio that is the from-scratch path for every new Roslyn Project instance whose options are not reusable — after every options recompute, since FSharpProjectOptions.AreSameForChecking compares only the Stamp — so a project with a few hundred references pays a few hundred stats per typecheck request.

This adds an optional getReferenceStamp: string -> DateTime to FromOptions. A host that already tracks the last-write times of its references (#20457 keeps them behind an IVsAsyncFileChangeEx2 watcher) supplies them; everyone else gets the same FileSystem.GetLastWriteTimeShim as before. The callback is threaded through the recursive call for FSharpReferencedProject.FSharpReference, so referenced projects' snapshots use it too.

No behaviour change for existing callers; the surface-area baseline gains the new optional parameter on the existing overload.

Checklist

• Test cases added — FSharpChecker/ProjectSnapshot.fs: the stamps of both the project and its referenced F# project come from the callback.
• Performance benchmarks added in case of performance changes — none; the change removes a stat per reference only when a host opts in, the default path is unchanged.
• Release notes entry updated — docs/release-notes/.FSharp.Compiler.Service/11.0.100.md.

[github-actions]

❗ Release notes required

You can open this PR in browser to add release notes: open in github.dev

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md