feat(engine): expose JobPartGeometry for reading snapshot material

Every plugin engine rebuilt part material from a snapshot by hand and
filtered only rapids, so all three kept counting scribe/etch marks as
material after 1b5e1b1 fixed it in the host. JobPartGeometry is the
validator's own reader made public: SpecialLayers.IsMaterial, validated
closed contours, material area, and TryRead returning null for unreadable
parts. The job validators now use it, so engines and validation agree.

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

OpenNest.Engine/Jobs/NestJobValidator.cs

@@ -48,7 +48,7 @@ public static class NestJobValidator
                 );
             try
             {
-                NestJobPlacementValidator.ValidateGeometry(part.Geometry);
+                _ = JobPartGeometry.Read(part.Geometry);
             }
             catch (ArgumentException exception)
             {