Round clip: true circle (Viewbox/1x1 ellipse) + grabbable corner handle (transparent source grid, non-hit-testable selection overlay); resize locks square in round mode

ai.md

@@ -110,8 +110,16 @@ C# / WPF (.NET 8) following MVVM:
   device never drowns the render thread.
 - **Clip/mirror:** per-Source `ClipShape` (Traditional rectangle / Round ellipse) + `IsMirrored`
   (`ScaleX = -1`). Rendered in the preview DataTemplate (Image for Traditional, `ImageBrush` inside an
-  `Ellipse` for Round); toggled from the source chip; persisted in the layout DB. Round hit-testing is the
-  same rectangle as Traditional (selection overlay is rectangular) — acceptable for now.
+  `Ellipse` for Round); toggled from the source chip; persisted in the layout DB.
+  - The Round `Ellipse` is wrapped in a `Viewbox Stretch="Uniform"` holding a `1x1` Grid, so it renders
+    as a true circle (diameter = the shorter source dimension) instead of an oval stretched to the
+    source rect — and the traditional `Image` keeps `UniformToFill` over the full rect.
+  - Resizing locks to a square (`_resizeAspect = 1`) while `ClipShape == Round`.
+  - **Hit-testing:** a `Grid` without `Background` only hit-tests where its children draw, so clicks in
+    the empty corners of a round clip fell through to `Window_PreviewMouseLeftButtonDown` and deselected
+    the source — making the corner handle ungrabbable. The source Grid carries `Background="Transparent"`
+    (whole rect draggable) and the `SelectionOverlay` (dashed border + corner dot) is
+    `IsHitTestVisible="False"` so it never intercepts the click.
 - **GPU posture:** webcam frames are CPU (GPU-agnostic; WPF hardware-presents the preview anyway). Hardware
   encoders (NVENC/AMF/QSV) matter for the encoder task, not capture. D3DImage GPU compositing is deferred
   to the encoder task.