Missing `Microsoft.ui.composition.h`

### Describe the bug

This header is NOT generated by `cppwinrt`, and is supposed to be packaged by the winui nuget package, but is somehow missing.

It is mainly used for Compositor interop with direct 2d.

### Why is this important?

This is broken since at least winui3 1.6.

### Steps to reproduce the bug

In a C++WinRT project, add `#include `, and you get the build error of missing `Microsoft.ui.composition.h`

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

1.8.260415005

### Windows version

Windows 10 (1809): Build 17763

### Additional context

_No response_

Start by reproducing the C++/WinRT project failure with #include using NuGet package version 1.8.260415005. Trace how Microsoft.ui.composition.h is generated or packaged, then verify that the header is present and the include builds successfully on the reported Windows environment.

Written by the indexing model from the issue text.