feat: add StripNestEngine with strip-based multi-drawing nesting

New NestEngineBase subclass that dedicates a tight strip to the
largest-area drawing and fills the remnant with remaining drawings.
Tries both bottom and left orientations, uses a shrink loop to find
the tightest strip, and picks the denser result.

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

OpenNest.Engine/StripNestResult.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest
+{
+    internal class StripNestResult
+    {
+        public List<Part> Parts { get; set; } = new();
+        public Box StripBox { get; set; }
+        public Box RemnantBox { get; set; }
+        public FillScore Score { get; set; }
+        public StripDirection Direction { get; set; }
+    }
+}