AppInstance.UnregisterKey() causes a failfast crash when called on a different thread than FindOrRegisterForKey(...) was called from

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### Describe the bug

When I call AppInstance.UnregisterKey() from a different thread than FindOrRegisterForKey(...) was called from, for me it consistently immediately crashes the process with a `0xc0000409` code. It seems to happen when trying to reset a mutex, `m_keyCreationMutexLock`, inside `UnregisterKey()`.

If this is intended/expected behavior, it should at least be called out in the documentation for `FindOrRegisterForKey` and `UnregisterKey`. This was very frustrating to debug.

### Steps to reproduce the bug

Minimal reproduction:
`

AppInstance.FindOrRegisterForKey("key");
Thread.Sleep(3000);
new Thread(() =>
{
AppInstance.GetCurrent().UnregisterKey();
}).Start();`

### Expected behavior

The key will be registered and unregistered, without crashing the process.

### Screenshots

_No response_

### NuGet package version

2.0.1

### Packaging type

Packaged (MSIX)

### Windows version

Windows 11 version 24H2 (26100, June 2025 Update)

### IDE

Other

### Additional context

_No response_

Start with the minimal reproduction using AppInstance.FindOrRegisterForKey and AppInstance.UnregisterKey, then investigate the cross-thread call and the m_keyCreationMutexLock reset described in the report. Done means the key can be registered and unregistered from the shown threads without a process crash, or the thread requirement is clearly documented for both APIs.

Written by the indexing model from the issue text.