feat(expand): plateview spacing expander

Grows part-to-part spacing of a selected group with the work area and
non-selected parts as hard boundaries. Doubling + bisection search over
the target spacing; Gauss-Seidel straight-line relaxation with anchor
mover policy (later-index selected part moves, first selection never).
Overlapping input is separated along penetration MTVs instead of being
rejected. Cancel/failure never mutates part positions. Clearance gains
BoundaryDistance for ring-pair gaps (part-in-cutout legality).

OpenNest.Core/Geometry/Clearance.cs

@@ -62,6 +62,24 @@ namespace OpenNest.Geometry
             return Separation(linesA, linesB);
         }
 
+        /// <summary>
+        /// Non-negative minimum boundary (edge-to-edge) distance between two rings
+        /// and the direction that translates <paramref name="a"/> away from
+        /// <paramref name="b"/> at the closest contact. Never tests overlap: a ring
+        /// contained in another (a part inside a cutout ring) still reports its true
+        /// gap. For signed material clearance use <see cref="Between"/>.
+        /// </summary>
+        public static ClearanceResult BoundaryDistance(Polygon a, Polygon b)
+        {
+            var linesA = a.ToLines();
+            var linesB = b.ToLines();
+
+            if (linesA.Count == 0 || linesB.Count == 0)
+                return new ClearanceResult(0, new Vector(1, 0));
+
+            return Separation(linesA, linesB);
+        }
+
         /// <summary>
         /// Minimum boundary distance between two non-overlapping rings and the
         /// direction that translates <paramref name="linesA"/> away from