[WinUI OSS] Scope development-only API cleanup for scheduled builds

## Summary

Scope cleanup needed where experimental IXP API usage affects scheduled or release-oriented build readiness.

## Context

IXP appears in public repository content, so it can be named at a high level. Engineers should identify which IXP usage is acceptable for public development and which usage blocks stable or scheduled build scenarios. Avoid private API rollout status, unpublished package details, or internal decision history.

## Work to do

- Identify IXP usage that affects scheduled or release-oriented build paths.
- Separate documentation/reference usage from build-blocking implementation usage.
- Estimate cleanup size and risk.
- Create implementation tasks for removals, guards, or replacements.
- Coordinate with #11279 and #11278.

## Definition of done

- Experimental IXP cleanup scope is understood.
- Required implementation follow-ups are identified.
- Risks are documented in public-safe language.
- Engineers can start investigation without private context in the issue body.