fix: update drawings in-place when editing in converter so parts reflect changes

EditDrawingsInConverter was replacing Drawing objects with new instances,
but Part.BaseDrawing is readonly — parts kept referencing the old drawings
with stale programs (e.g. etch lines that were removed). Now matches by
name and updates existing drawings in-place, then refreshes all parts.

Also fixes Part.Update() which applied rotation backwards and was missing
UpdateBounds() and lead-in state reset.

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

OpenNest.Core/Part.cs

@@ -190,7 +190,14 @@ namespace OpenNest
         {
             var rotation = Rotation;
             Program = BaseDrawing.Program.Clone() as Program;
-            Program.Rotate(Program.Rotation - rotation);
+
+            if (!Math.Tolerance.IsEqualTo(rotation, 0))
+                Program.Rotate(rotation);
+
+            HasManualLeadIns = false;
+            LeadInsLocked = false;
+            CuttingParameters = null;
+            UpdateBounds();
         }
 
         /// <summary>