[Parley] Flowchart layout direction: Add left-to-right option

Summary

The native flowchart view currently displays the dialog tree top-to-bottom (root at top). Users prefer root node on the left with conversation flowing left-to-right.

Technical Details

• AvaloniaGraphControl uses MSAGL (Microsoft Automatic Graph Layout) internally
• MSAGL supports LayerDirection.LR via SugiyamaLayoutSettings.Transformation = PlaneTransformation.Rotation(Math.PI / 2)
• AvaloniaGraphControl (v0.6.1) does NOT expose this property

Potential Solutions

1. Fork AvaloniaGraphControl - Add LayerDirection dependency property that maps to MSAGL settings
2. Submit upstream PR - Request this feature in https://github.com/Oaz/AvaloniaGraphControl
3. Custom layout adapter - Override the layout generation in FlowchartGraphAdapter

References

• MSAGL LayerDirection: https://csharp.hotexamples.com/examples/Microsoft.Msagl.Layout.Layered/SugiyamaLayoutSettings/-/php-sugiyamalayoutsettings-class-examples.html
• AvaloniaGraphControl: https://github.com/Oaz/AvaloniaGraphControl

Epic

Part of Epic #325 - Native Flowchart View