Port Streams from 7.x

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

## Description

There are several operations that apps need commonly to do against their APPX, or from the Internet that are not easy. This helper class wraps up some of the most common operations we need in multiple apps.

## 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

---

## 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/streams

アーカイブされた Streams ドキュメントと 7.x ソースから始め、次に Porting Criteria を使用して不足している WinUI と Uno の範囲を定義します。名前空間、マルチターゲット、サンプル、概念ドキュメント、API サーフェス、テストを確認します。完了とは、Porting Basics、Quality Evaluation、Closing のチェックリストがすべて完了していることを意味します。

索引モデルが issue の本文から書いたものです。