feat(rectangles): avoid clamp zones in sheet packing

Block each clamp's keep-out into the max-rects free list before any
packing: same split logic as placements, not a used item (no contact
score, no material). The blocked rectangle accounts for the inflated
item model exactly - material keeps the bottom-edge clearance from the
clamp without paying an extra part-spacing tax; degenerate spans
collapse to the zone centerline (conservative, never permissive).

Zero-clamp sheets skip blocking entirely: three fixture layout hashes
(rectangle grid, mixed shapes, multi-sheet overflow), captured from the
unchanged packer at a3f1124, stay bit-identical (C2). With clamps:
placements never intrude, demand lands between two clamps when it fits,
a full-width front rail makes an otherwise feasible job Incomplete, and
a far-away zone leaves the layout hash equal to no-clamps.

Rectangles lane only; Irregular parity is Slice 2.

OpenNest.Engine/RectanglePacking/MaxRectsSheet.cs

@@ -91,6 +91,20 @@ internal sealed class MaxRectsSheet
 
     /// <summary>Commits an item and splits every free rectangle it intersects.</summary>
     public void Place(Rect item)
+    {
+        Split(item);
+        used.Add(item);
+    }
+
+    /// <summary>
+    /// Carves a keep-out rectangle (a machine clamp's inflated exclusion, for example) out
+    /// of the free space before any packing: same split logic as placements, but not a used
+    /// item - it consumes no material and contributes nothing to contact scoring. The
+    /// rectangle may extend past the bin; only interior intersections count.
+    /// </summary>
+    public void Block(Rect keepOut) => Split(keepOut);
+
+    private void Split(Rect item)
     {
         var next = new List<Rect>(free.Count + 8);
         foreach (var f in free)
@@ -111,7 +125,6 @@ internal sealed class MaxRectsSheet
         }
         free.Clear();
         free.AddRange(Prune(next));
-        used.Add(item);
     }
 
     private static List<Rect> Prune(List<Rect> rects)