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.Engine/Fill/PartBoundary.cs

@@ -33,7 +33,7 @@ namespace OpenNest.Engine.Fill
 
             if (perimeter != null)
             {
-                var offsetEntity = perimeter.OffsetEntity(spacing, OffsetSide.Left) as Shape;
+                var offsetEntity = perimeter.OffsetOutward(spacing);
 
                 if (offsetEntity != null)
                 {