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/README.md

@@ -15,9 +15,9 @@ Every placement decision (which part, which rotation, where, on which sheet) com
   angles plus the two orientations that axis-align the minimum-area bounding rectangle
   (`RotatingCalipers`); for sweeps, up to 8 evenly spaced legal steps. Point-symmetric duplicates are dropped.
 - Each orientation gets a **footprint**: outline inflated (miter joins, so it contains the exact
-  round offset) by `(spacing + 0.022) / 2 + chordTolerance`. Two parts respect the spacing
-  when their footprints don't overlap. The 0.022 covers validators that polygonize arcs
-  circumscribed at 0.01 per side, plus Clipper's 1e-4 grid.
+  round offset) by `(spacing + 0.003) / 2 + chordTolerance`. Two parts respect the spacing
+  when their footprints don't overlap. The 0.003 covers `NestValidator` flattening each arc
+  within 0.001 of true (0.002 for a pair), plus Clipper's 1e-4 grid on both sides.
 - **No-fit polygons** between footprints come from Clipper2 Minkowski sums: an O(n+m)
   edge merge for convex pairs, and for concave pairs the boundary sweep ∪ (A + p₀) ∪ (−B + a₀).
   The last two terms cover "B inside A" and "B swallows A". NFPs are cached per orientation pair.
@@ -83,9 +83,10 @@ The engine reports as `Opus55NestingEngine`.
 ## Known limitations
 
 - **No part-in-part:** holes are treated as solid, so small parts never nest inside cutouts.
-- **Clearance padding:** gaps are ~0.022 (plus up to the chord tolerance) wider than the
-  required spacing, to stay valid under circumscribed-polygon validators. That's negligible in mm
-  and about 0.02" in inches. The constants are absolute and assume job units near inch/mm scale.
+- **Clearance padding:** gaps are ~0.003 (plus up to the chord tolerance) wider than the
+  required spacing, to stay valid under `NestValidator`'s 0.001 arc flattening. The margin was
+  0.022 while it assumed a 0.01 validator tolerance. The constants are absolute and assume job
+  units near inch/mm scale.
 - **Rotation coverage:** `Automatic` parts try at most 8 orientations (fewer when a job has many
   distinct parts: `48 / partCount`, minimum 2). Free-angle rotations aren't explored beyond the MBR alignment.
 - **Greedy core:** there is no order/permutation search. The variants and tail re-plan are the only