TextBlock can't be right aligned

### Describe the bug

I'm trying to make a `TextBlock` be right aligned, where the text is always pinned to the right edge of the `TextBlock` and as the `TextBlock` is resized, trimming of the `TextBlock` occurs on the left side.

It sounds like `FlowDirection` of `RightToLeft` should be the property to set, along with setting `TextReadingOrder` to `UseFlowDirection` but this doesn't seem to work. The `TextBlock` still behaves the same way

### Steps to reproduce the bug

1. Create a blank C# app
2. Add this `TextBlock` to the `MainWindow.xaml`:

```

```

### Expected behavior

The end of the TextBlock is visible "stick to the right side and be truncated at the left"

### Screenshots


### NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

If there is a different way to accomplish this not through these APIs I'd be happy to do this

Start with the TextBlock reproduction in MainWindow.xaml from the issue, using the stated TextAlignment, TextReadingOrder, FlowDirection, Width, and long text. Confirm how resizing and truncation behave in the blank C# app, then trace the relevant WinUI TextBlock behavior. Done means the expected text remains pinned to the right edge and is truncated on the left.

Written by the indexing model from the issue text.