Publish NavigableContainer's cases

[xperiandri]

NavigateTo.GetNavigableItems returns items a consumer can read but not rebuild: the signature seals
NavigableContainer and hides its File and Container cases. That is enough to display a result, and not
enough to carry one out of the process that produced it. Visual Studio's Navigate To is to keep each file's
navigable items on disk between sessions, as Roslyn keeps its syntax index, and reading them back means
constructing containers.

The change

• NavigableContainer publishes File of fileName: string and Container of info: NavigableContainerInfo. Its
  members Type, FullName and Name are unchanged.

• Container carried a three-element tuple, containerType * nameParts * parent. Published as it was, the tuple
  would be fixed into the API, so it becomes a record with named fields first:
  NavigableContainerInfo { ContainerType; NameParts; Parent }.

• The record is a [<Struct>]. A struct record lies inside the case exactly as the tuple's fields did — one
  allocation per container — where a reference record adds an object per container and an indirection on every
  access. Building a file and three nested containers a million times (x64, .NET 11, fsi --optimize+):

  Container payload	bytes per chain
  tuple (before)	144
  [<Struct>] record	144
  reference record	216

  A struct union is not an option: Parent is recursive.

The change is additive: the surface-area baseline only gains lines. NavigableContainer keeps its structural
equality and comparison, which the struct record derives as well.

SurfaceAreaTest passes against the updated FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl. The
consumer, a persistent cache for Navigate To in FSharp.Editor, is a separate PR that builds on this one.

🤖 Generated with Claude Code

[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

[github-actions]

🔍 Tooling Safety Check — Affects-Design-Time
Affects-Design-Time: Changes navigable compiler-service data exposed to IDE features.

Generated by PR Tooling Safety Check · gpt56 3.1M · ◷

[T-Gro]

🤖 🕵️ AI review — verify independently.

[T-Gro]

🤖 🕵️ [P2] Ordered-container comparisons now box each NavigableContainerInfo — 120 bytes per comparison at depth three, versus 0 with the previous representation.

open System
open System.Collections.Generic
open FSharp.Compiler.EditorServices

let chain file =
    [1..3] |> List.fold (fun parent _ ->
        NavigableContainer.Container {
            ContainerType = NavigableContainerType.Module
            NameParts = ["M"]
            Parent = parent
        }) (NavigableContainer.File file)

let a, b = chain "a.fs", chain "b.fs"
let comparer = Comparer<NavigableContainer>.Default
for _ in 1..10000 do comparer.Compare(a, b) |> ignore

let allocated =
    let before = GC.GetAllocatedBytesForCurrentThread()
    for _ in 1..100000 do comparer.Compare(a, b) |> ignore
    GC.GetAllocatedBytesForCurrentThread() - before
printfn "%d bytes" allocated // 12000000 bytes