feat: add NFP-based mixed-part autonesting

Implement geometry-aware nesting using No-Fit Polygons and simulated
annealing optimization. Parts interlock based on true shape rather than
bounding boxes, producing tighter layouts for mixed-part scenarios.

New types in Core/Geometry:
- ConvexDecomposition: ear-clipping triangulation for concave polygons
- NoFitPolygon: Minkowski sum via convex decomposition + Clipper2 union
- InnerFitPolygon: feasible region computation for plate placement

New types in Engine:
- NfpCache: caches NFPs keyed by (drawingId, rotation) pairs
- BottomLeftFill: places parts using feasible regions from IFP - NFP union
- INestOptimizer: abstraction for future GA/parallel upgrades
- SimulatedAnnealing: optimizes part ordering and rotation

Integration:
- NestEngine.AutoNest(): new public entry point for mixed-part nesting
- MainForm.RunAutoNest_Click: uses AutoNest instead of Pack
- NestingTools.autonest_plate: new MCP tool for Claude Code integration
- Drawing.Id: auto-incrementing identifier for NFP cache keys
- Clipper2 NuGet added to OpenNest.Core for polygon boolean operations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Drawing.cs

@@ -1,5 +1,6 @@
 using System.Drawing;
 using System.Linq;
+using System.Threading;
 using OpenNest.CNC;
 using OpenNest.Converters;
 using OpenNest.Geometry;
@@ -8,6 +9,7 @@ namespace OpenNest
 {
     public class Drawing
     {
+        private static int nextId;
         private Program program;
 
         public Drawing()
@@ -22,6 +24,7 @@ namespace OpenNest
 
         public Drawing(string name, Program pgm)
         {
+            Id = Interlocked.Increment(ref nextId);
             Name = name;
             Material = new Material();
             Program = pgm;
@@ -29,6 +32,8 @@ namespace OpenNest
             Source = new SourceInfo();
         }
 
+        public int Id { get; }
+
         public string Name { get; set; }
 
         public string Customer { get; set; }