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/Geometry/Intersect.cs

@@ -219,6 +219,14 @@ namespace OpenNest.Geometry
         }
 
         internal static bool Intersects(Line line1, Line line2, out Vector pt)
+        {
+            if (!IntersectsUnbounded(line1, line2, out pt))
+                return false;
+
+            return line1.BoundingBox.Contains(pt) && line2.BoundingBox.Contains(pt);
+        }
+
+        internal static bool IntersectsUnbounded(Line line1, Line line2, out Vector pt)
         {
             var a1 = line1.EndPoint.Y - line1.StartPoint.Y;
             var b1 = line1.StartPoint.X - line1.EndPoint.X;
@@ -240,7 +248,7 @@ namespace OpenNest.Geometry
             var y = (a1 * c2 - a2 * c1) / d;
 
             pt = new Vector(x, y);
-            return line1.BoundingBox.Contains(pt) && line2.BoundingBox.Contains(pt);
+            return true;
         }
 
         internal static bool Intersects(Line line, Shape shape, out List<Vector> pts)