Refactor PieControl into focused visual, animation, and selection helpers

[Copilot]

PieControl.xaml.cs had accumulated rendering support, animation behavior, and keyboard selection logic in one file, making it harder to reason about without changing UI behavior. This refactor splits those responsibilities into focused helpers while keeping the control API, bindings, and menu interactions unchanged.

• What moved

  • Extracted slice and center visual models into PieSliceVisual and PieCenterVisual
  • Extracted brush/opacity/click-scale animation behavior and reduced-motion handling into PieAnimationService
  • Extracted keyboard selection state, directional selection, wrapping, and invalid-selection reset into PieSelectionController

• What stayed in PieControl

  • WPF event wiring
  • dependency properties and existing public events
  • render orchestration and existing layout/visual-builder integration

• Behavior-preserving wiring changes

  • PieControl now delegates keyboard navigation to PieSelectionController and visual transitions to PieAnimationService
  • Existing events remain unchanged:
    • SliceClicked
    • CenterClicked
    • SliceEditRequested
    • CenterContextMenuRequested

• Coverage added

  • Added focused pure tests for PieSelectionController covering:
    • first directional selection choosing the nearest slice by angle
    • left/up backward wrapping
    • right/down forward wrapping
    • resetting selection when the selected index is no longer present

_selectionController.HandleArrowKey(key, GetSelectionItems());

if (_selectionController.SelectedIndex == PieSelectionController.NoSelection)
{
    return;
}

_animationService.ApplyBrushColor(
    sliceVisual.FillBrush,
    _renderState.HoverColor,
    animate,
    _renderState.HoverDuration,
    _renderState.StandardEasing);