[WinUI OSS] Audit build mode parameterization

Summary

Audit build mode parameterization so workflows explicitly choose the intended public, scheduled, or release-oriented behavior.

Context

The ADO title references IsInternalBuild, but public issue text should focus on explicit build mode selection. Engineers should look for places where build behavior is inferred from source layout, file presence, or environment instead of being intentionally configured.

Work to do

• Identify build mode switches and implicit behavior in public build files.
• Determine where explicit parameters are needed.
• Confirm public PR builds select public-safe behavior.
• Confirm scheduled/release-oriented builds have clear configuration paths.
• Coordinate with package restore, MUXFinalRelease, and WindowsAppSDK integration tasks.

Definition of done

• Build mode selection points are known.
• Public versus scheduled behavior is explicit or tracked.
• Follow-up implementation tasks are created for risky implicit behavior.
• Public issue text does not expose private mode names beyond safe, high-level concepts.