FCS: fix races that made the background builder repeat work

[auduchinok]

• IncrementalBuilder: read and replace the state under the semaphore, so concurrent requests after a file change no longer each build their own chain of bound models, and concurrent NotifyFileChanged calls no longer lose each other's notification.
• BackgroundCompiler: replace the builder node only if nobody else already did, so invalidated references create one builder instead of one per concurrent caller.
• BackgroundCompiler: memoize parse results as a node created under the cache lock, so concurrent parses of the same source share one parse.
• Add a stress test: 50 concurrent project checks after a file change type-checked every file 10 times before, once now.

[github-actions]

❗ Release notes required

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---

✅ 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

[T-Gro]

🤖 🕵️ [P2] Unchanged files reparse after GC even while their parse results are retained. With FCS_ParseFileCacheSize=2 set before launch, this prints 0; true on base and 1; false on head: the weak entry targets a GraphNode that the retained result does not keep alive. Preserve weak reuse of completed results alongside in-flight sharing.

open System
open System.IO
open FSharp.Compiler.CodeAnalysis
open FSharp.Compiler.Text

let repro () =
    let checker = FSharpChecker.Create(useTransparentCompiler = false)
    let files = Array.init 3 (fun i -> Path.GetFullPath $"CacheProbe{i}.fs")
    let options = { FSharpParsingOptions.Default with SourceFiles = files }
    let source = SourceText.ofString "module CacheProbe\nlet value = 1"
    let parse i = checker.ParseFile(files[i], source, options, cache = true) |> Async.RunSynchronously
    let retained = Array.init 3 parse
    GC.Collect()
    GC.WaitForPendingFinalizers()
    GC.Collect()
    let before = FSharpChecker.ActualParseFileCount
    let again = parse 0
    printfn "%d; %b" (FSharpChecker.ActualParseFileCount - before)
        (Object.ReferenceEquals(retained[0], again))
    GC.KeepAlive retained

repro ()