fix(opus55): cut clearance margin to 0.003 and drop etch marks from geometry

The 0.022 margin assumed validators flatten arcs at 0.01; NestValidator
uses 0.001, so a pair can read at most 0.002 closer than true. 0.003
covers that plus the 1e-4 Clipper grid, returning ~0.019 per gap. Arc,
disc, obround and zero-spacing tests still pass.

Part geometry filtered only rapids, so scribe/etch moves counted as
material - the bug OpenNest fixed in 1b5e1b1. Use SpecialLayers.IsMaterial.

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

OpenNest.Engine.Opus55/Opus55NestingEngine.cs

@@ -20,11 +20,15 @@ namespace OpenNest.Engine.Opus55;
 public sealed class Opus55NestingEngine : INestingEngine
 {
     /// <summary>
-    /// Extra clearance beyond the stock's part spacing, in job units. Validators polygonize arcs
-    /// circumscribed at 0.01 per side, so two tangent true arcs can read as up to 0.02 closer
-    /// than they are; the rest absorbs Clipper's 1e-4 grid and inner-fit clamping.
+    /// Extra clearance beyond the stock's part spacing, in job units. Footprints already contain
+    /// the true outline grown by half the clearance (chord tolerance is added separately), so this
+    /// only has to cover the validator's side: NestValidator flattens each perimeter within
+    /// 0.001 of the true arc (OutlineTolerance), so two true arcs can read up to 0.002 closer
+    /// than they are. The remaining 0.001 absorbs the 1e-4 Clipper grid on both sides (footprint
+    /// offset, Minkowski union, free-region difference, the validator's own offset); the
+    /// validator's round-join chord error (0.0001) only ever makes it more lenient.
     /// </summary>
-    internal const double ClearanceMargin = 0.022;
+    internal const double ClearanceMargin = 0.003;
 
     /// <summary>Strategy variants, tried in order: (front direction, area exponent beta).</summary>
     private static readonly (PackAxis Axis, double Beta)[] Variants =