Building multi-project WinUI solutions takes ages even after a tiny change because of PRI recalculation

### Describe the bug

Even if I change a tiny thing (code or XAML) in the referenced class library project and try running the app, its build takes minutes.
See binlogs in the report https://developercommunity.visualstudio.com/t/Building-WinUI-project-takes-ages-even-a/10932418

### Steps to reproduce the bug

I can give an access to the affected solution but I don't know how to reproduce on a smaller solution

### Expected behavior

PRIs are not recalculated and project builds and runs instantaneously

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.7.3: 1.7.250606001

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start with the linked Developer Community report and its build logs, focusing on the WinUI 3 and Windows App SDK 1.7.3 build behavior after a class library code or XAML change. Determine which build step recalculates PRIs and under what conditions; done means an incremental build avoids unnecessary PRI recalculation and runs promptly.

Written by the indexing model from the issue text.