Port ObjectStorage from 7.x

**Source**: Microsoft.Toolkit.Uwp.UI.Helpers (7.x)
**Destination**: CommunityToolkit.WinUI.Helpers (8.x) — not yet ported

## Description

Helps you handle storage of generic objects within UWP applications, both locally and across all devices (roaming).

## Prerequisites

None identified.

## Porting Basics

- [ ] Namespace updated to `CommunityToolkit.WinUI.*`
- [ ] Code for UWP, WinUI3 and Uno Platform consolidated
- [ ] Desired MultiTargets configured
- [ ] Sample code migrated
- [ ] Concept docs migrated
- [ ] Tests migrated and passing

## Quality Evaluation

See [Porting Criteria](https://github.com/CommunityToolkit/Windows/wiki/Component-Quality-Evaluation) for detailed evaluation guidance.

- [n/a] UX: Accessibility reviewed
- [n/a] UX: Visual design reviewed
- [n/a] UX: Interaction patterns appropriate
- [ ] DX: API surface reviewed
- [ ] DX: API flexibility appropriate
- [ ] DX: Documentation updated
- [ ] MX: Breaking changes documented
- [ ] MX: Domain isolation appropriate
- [ ] MX: Test coverage adequate

## Tracking Gaps

*No gaps identified — porting work not yet started.*

## Closing

- [ ] All Porting Basics complete
- [ ] Quality Evaluation reviewed
- [ ] Gaps filed as sub-issues

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## 7x Documentation Inventory

**Docs ported to 8x**: n/a

**Docs not in 8x**: n/a

## 7x Source Inventory

**Source already ported to 8x**: n/a

**Source not ported to 8x**: n/a

## Notes

- Not currently planned for porting. Open to community interest.
- **Archive redirect**: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/windows/archive/objectstorage

Beginne mit der archivierten ObjectStorage-Dokumentation und den Portierungskriterien und ermittle dann den fehlenden Quellcode, das fehlende Beispiel, die fehlende Dokumentation, die Ziel-Frameworks und die Tests, da dieses Issue keine vorhandenen 8.x-Dateien nennt. Als abgeschlossen gilt die vollständige Bearbeitung der Checklisten Porting Basics und Quality Evaluation, die Migration der Tests mit bestandener Abdeckung sowie die Dokumentation etwaiger Lücken.

Vom Indexierungsmodell aus dem Issue-Text verfasst.