Plan viewer: a minimap of the whole plan #4580

Part of #4511, plan viewer parity with PerformanceStudio (PS). The viewer control is shared by the Darling Viewer and Lite, so each change lands in both.

PS erikdarlingdata/PerformanceStudio@a8aa163 (later refactored into PlanViewerControl.Minimap.cs, 523 lines) adds a toggle button that opens a resizable (160–500px, persisted across plans in-memory) corner panel: a scaled-down Canvas showing (1) translucent colored rectangles behind each root-child subtree (8-color cycle), (2) elbow connectors with the same log(rows)-scaled thickness and accuracy-ratio coloring as the main canvas, (3) small per-node rectangles (red tint for "expensive" nodes, each showing the operator icon at ~70% size), (4) a semi-transparent viewport box that tracks scroll/zoom/pan, and (5) click-to-center / double-click-to-zoom-and-select on a minimap node. PM has no Minimap symbol anywhere (grep -rl Minimap returns nothing).

WPF design sketch: a Grid-hosted Canvas (MinimapCanvas) inside a Border overlay anchored top-left of the plan ScrollViewer's AdornerLayer or a sibling Grid cell with Panel.ZIndex; reuse WPF Thumb for the resize grip instead of Avalonia's pointer-drag math; recompute scale = min(canvasW/PlanCanvas.Width, canvasH/PlanCanvas.Height) on each RenderMinimap(); reuse PlanLayoutEngine.NodeWidth/GetNodeHeight (present in PM too) for bounds; the viewport box binds to ScrollViewer.HorizontalOffset/VerticalOffset/ViewportWidth/ViewportHeight via ScrollChanged. Node color/icon reuse PlanIconMapper.GetIcon.

At 08f3e3c4e. PS reference: its dev branch at 85492a1.