F# VS editor: reduce background CPU load by gating expensive analyzers to the active document only

[xperiandri]

Fixes #20114

Summary

CPU profiling of devenv.exe (VS Insiders + F# extension) showed ThreadPoolWorkQueue.Dispatch at 72 % total / 59 % self, driven by continuous F# background analysis running for all open tabs, not just the active one.

This PR implements the highest-impact P0 fixes from docs/ide/background-activity-minimization-plan.md:

Changes

New: ActiveDocumentDetection module

vsintegration/src/FSharp.Editor/Diagnostics/ActiveDocumentDetection.fs

Shared helper that uses IVsMonitorSelection / IVsWindowFrame to determine whether a Roslyn Document is the currently focused VS editor tab.

• tryGetActiveDocumentMoniker — returns string voption (the active file path, or ValueNone)
• isActiveDocument — returns bool; falls back to true (= don't suppress) when detection fails, so analysis is never silently lost

Gated analyzers / services

Component	Before	After
UnusedOpensDiagnosticAnalyzer	Used inline shell detection	Refactored to shared ActiveDocumentDetection module
UnusedDeclarationsAnalyzer	No gating — ran full typecheck for every open tab	Gated to active document
SimplifyNameDiagnosticAnalyzer	No gating — 3 concurrent typechecks for all open tabs	Gated to active document
FSharpInlayHintsService	No gating — N−1 unnecessary typechecks	Gated to active document

Behavior

• Non-active documents receive no background diagnostics/hints while out of focus.
• Switching to a tab immediately triggers fresh analysis for that tab.
• This matches C#'s default behavior (BackgroundAnalysisScope.VisibleFilesAndOpenFilesWithPreviouslyReportedDiagnostics).
• Roslyn clears stale diagnostics automatically when a document becomes active.

Not in scope (tracked in the plan)

• BackgroundAnalysisScope default change (item App.config in up-to-date check causes rebuilds #11) — needs a separate option key
• Compilation.Emit version-stamp cache (item Error while installing Visual F# Tools (05.01.2015) #5)
• hasDependentVersionChanged async fix (item Method not fournd: SystemCollections.Generic IEqualityComparer'1 #6)
• ClassificationService stale-result path (item Existing F# projects won't open if only VS2015 is installed #4)
• Script updateProjectOptions debounce (item Intellisense does not appear off of generic type #8)
• FSharpProjectOptionsReactor priority queue (item Explicit field declaration generating property in F# classes #7)