[WinUI OSS] Confirm source synchronization and build/release precedent

## Summary

Confirm the WinAppSDK precedent for GitHub source synchronization and build/release flow, then decide what applies to WinUI.

## Context

The public WinUI repo includes WindowsAppSDK references such as `docs/winappsdk-overview.md`, `build/nuspecs/Microsoft.WindowsAppSDK.WinUI.nuspec`, and `packaging/build/cmake/microsoft.windowsappsdk.winui-config.cmake`. Engineers should use public artifacts and team knowledge to clarify how WinUI should align with WindowsAppSDK integration.

## Work to do

- Identify the public-safe WinAppSDK source/build/release pattern relevant to WinUI.
- Confirm where GitHub source should be authoritative.
- Document how WinUI component output flows toward WindowsAppSDK integration.
- Track differences that require WinUI-specific follow-up work.
- Avoid private repo links, private release orchestration details, or internal-only schedules.

## Definition of done

- WinAppSDK precedent is understood.
- WinUI alignment decisions are documented.
- Follow-up tasks exist for gaps.
- Public text remains safe and actionable.