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.Engine/INestOptimizer.cs

@@ -0,0 +1,38 @@
+using System.Collections.Generic;
+using System.Threading;
+using OpenNest.Geometry;
+
+namespace OpenNest
+{
+    /// <summary>
+    /// Result of a nest optimization run.
+    /// </summary>
+    public class NestResult
+    {
+        /// <summary>
+        /// The best sequence found: (drawingId, rotation, drawing) tuples in placement order.
+        /// </summary>
+        public List<(int drawingId, double rotation, Drawing drawing)> Sequence { get; set; }
+
+        /// <summary>
+        /// The score achieved by the best sequence.
+        /// </summary>
+        public FillScore Score { get; set; }
+
+        /// <summary>
+        /// Number of iterations performed.
+        /// </summary>
+        public int Iterations { get; set; }
+    }
+
+    /// <summary>
+    /// Interface for nest optimization algorithms that search for the best
+    /// part ordering and rotation to maximize plate utilization.
+    /// </summary>
+    public interface INestOptimizer
+    {
+        NestResult Optimize(List<NestItem> items, Box workArea, NfpCache cache,
+            Dictionary<int, List<double>> candidateRotations,
+            CancellationToken cancellation = default);
+    }
+}