feat: overhaul SplitDrawingForm — EntityView, draggable feature handles, UI fixes

- Replace raw Panel with EntityView (via SplitPreview subclass) for proper
  zoom-to-point, middle-button pan, and double-buffered rendering
- Add draggable handles for tab/spike positions along split lines; positions
  flow through to WeldGapTabSplit and SpikeGrooveSplit via SplitLine.FeaturePositions
- Fix OK/Cancel buttons hidden off-screen by putting them in a bottom-docked panel
- Fix DrawControl not invalidating on resize
- Swap plate Width/Length label order, default edge spacing to 0.5
- Rename tab labels: Tab Width→Tab Length, Tab Height→Weld Gap, default count 2
- Spike depth now calculated (read-only), groove depth means positioning depth
  beyond spike tip (default 0.125), converted to total depth internally
- Set entity layers visible so EntityView renders them

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/Splitting/SpikeGrooveSplit.cs

@@ -26,15 +26,19 @@ public class SpikeGrooveSplit : ISplitFeature
         var isVertical = line.Axis == CutOffAxis.Vertical;
         var pos = line.Position;
 
-        // Place pairs evenly: one near each end, with margin
-        var margin = extent * 0.15;
+        // Use custom positions if provided, otherwise place evenly with margin
         var pairPositions = new List<double>();
-        if (pairCount == 1)
+        if (line.FeaturePositions.Count > 0)
+        {
+            pairPositions.AddRange(line.FeaturePositions);
+        }
+        else if (pairCount == 1)
         {
             pairPositions.Add(extentStart + extent / 2);
         }
         else
         {
+            var margin = extent * 0.15;
             var usable = extent - 2 * margin;
             for (var i = 0; i < pairCount; i++)
                 pairPositions.Add(extentStart + margin + usable * i / (pairCount - 1));