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/StripDirection.cs

@@ -0,0 +1,8 @@
+namespace OpenNest
+{
+    public enum StripDirection
+    {
+        Bottom,
+        Left
+    }
+}