fix(engine): allow 0.0005 spacing slack in layout validation

Layouts placed exactly at the part spacing can land ~1e-4 short once
rotated, rounded (e.g. PEP's 4-decimal exports) and snapped to the
Clipper grid, so both validators rejected layouts that were correct in
practice. NestTolerances.SpacingSlack (0.0005, far below anything a
cutting machine resolves) is now subtracted from the spacing by
NestLayoutCheck's inflation and NestJobPlacementValidator's edge-distance
check. The frozen LegacyNestValidator takes the same rule so the
equivalence tests keep comparing like with like.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Engine/Jobs/NestTolerances.cs

@@ -12,6 +12,12 @@ public static class NestTolerances
     /// perimeter arcs and inscribes cutouts; the placement validator uses inscribed arcs.</summary>
     public const double ValidationOutline = 0.001;
 
+    /// <summary>How far under the part spacing a gap may fall and still pass both validators.
+    /// Layouts placed exactly at the spacing land up to ~1e-4 short once rotated, rounded
+    /// coordinates (e.g. PEP's 4-decimal exports) meet the Clipper grid; 0.0005 covers that
+    /// with margin and is far below anything a cutting machine can resolve.</summary>
+    public const double SpacingSlack = 0.0005;
+
     /// <summary>Clipper decimal precision (a 1e-4 coordinate grid).</summary>
     public const int ClipperPrecision = ClipperBridge.Precision;