Plan viewer: colour each plan edge by how far actual rows differ from the estimate #4579

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.

"Colored links" is edge (elbow-connector) coloring by the child operator's estimate-vs-actual row-count ratio, replacing the flat EdgeBrush. PS erikdarlingdata/PerformanceStudio@70d28e6 (PlanViewerControl.Rendering.cs:379-447, GetLinkColorBrush(PlanNode, double divergenceLimit)): only colors edges on actual plans (estimated plans keep neutral EdgeBrush); computes accuracyRatio = ActualRows/EstimateRows; within [1/limit, limit] stays neutral; outside that band climbs through Light Orange → Fluo Orange → Fluo Red (overestimated, i.e. fewer actual rows) or Blue → Light Blue → Fluo Blue (underestimated) at limit, limit×10, limit×100 thresholds. limit is a user setting, AccuracyRatioDivergenceLimit (default 10), loaded once per render, floored at 2.0. The minimap (D2) reuses the same function.

PM has only a single static EdgeBrush (PlanViewerControl.xaml.cs:39) — no accuracy-based coloring, no setting. WPF equivalent: a pure helper GetLinkColorBrush(PlanNode child, double divergenceLimit) returning one of 7 brushes, called from wherever PM builds its elbow-connector Path/Line (need to locate PM's edge-drawing method in PlanViewerControl.Rendering.cs — not yet read in this pass); add AccuracyRatioDivergenceLimit to PM's settings service.

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