[WinUI OSS] Track stable-build compatibility for public development

## Summary

Track cleanup needed where experimental IXP API usage affects stable public build readiness.

## Context

The public repo already contains IXP references in design notes, build scripts, and native source. This task should identify which usages are acceptable public references versus usages that create risk for stable builds. The issue should stay focused on the stable-build impact and avoid private API rollout details.

## Work to do

- Identify IXP API usage that affects stable public builds.
- Separate harmless public references from build-blocking or release-blocking usage.
- Determine whether affected code should use stable alternatives, build guards, or follow-up work.
- Coordinate with dependency cleanup and build/release model tasks.
- Keep private API availability, private timelines, and private dependency details out of public text.

## Definition of done

- Stable-build IXP compatibility risks are identified.
- Required replacements or guards are tracked.
- Public build behavior is not blocked by experimental-only API assumptions.
- Engineers can locate the affected category of work without needing private context.