refactor(gpt6astra): use host geometry, tolerances and layout checks

Gpt6Astra reverse-engineered the validator: hand-tuned paddings and a
copied check sequence (ValidationOverlap) to match its rounding. It now
reads parts with JobPartGeometry, takes clearance from NestTolerances,
checks candidates with NestLayoutCheck.Clears, and assembles results with
NestJobResultBuilder and NestJobCost; its tests use the shared kit. Its
contact search, beam search and extra Automatic angles are unchanged.

Synthetic benchmark (5 jobs, salvage 0.5): all valid, 2 sheets each,
cost 5574.07 -> 5470.07; time 1871 -> 2400 ms from the stricter shared
check on arc-heavy jobs.

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

OpenNest.Engine.Gpt6Astra/ContactGeometry.cs

@@ -1,5 +1,6 @@
 using Clipper2Lib;
 using OpenNest.Geometry;
+using OpenNest.Engine.Jobs;
 using M = System.Math;
 
 namespace OpenNest.Engine.Gpt6Astra;
@@ -47,7 +48,9 @@ internal sealed class ContactGeometry
         }
         token.ThrowIfCancellationRequested();
         var delta = spacing + stationary.ContactError + moving.ContactError
-            + (stationary.Curved || moving.Curved ? 0.003 : spacing > 0 ? 0.0003 : 0);
+            + (stationary.Curved || moving.Curved
+                ? NestTolerances.SafeClearanceMargin(NestTolerances.ValidationOutline)
+                : spacing > 0 ? NestTolerances.SafeClearanceMargin(0) : 0);
         if (delta > 0) paths = Clipper.InflatePaths(paths, delta, JoinType.Round,
             EndType.Polygon, 2, GeometryPrecision.Digits, 0.00001);
         // Bound cache residency for jobs with many distinct rotation pairs.