fix(engine): enforce whole-job safety invariants

Task 5 of the whole-job engine API: add a geometry safety gate that
validates every candidate trial before the runner commits accounting.

- NestJobPlacementValidator: closed-contour validity, rotation-policy
  compliance, work-area containment per quadrant, hole-aware material
  overlap, and required part spacing. Overlap is interior-only, so
  zero-clearance edge/corner contact remains a valid placement.
- NestJobValidator: route candidate validation through the geometry
  gate; reject unusable/unclosed/degenerate contours up front.
- NestJobRunner: wrap candidate evaluation in a progress bridge that
  tags legacy engine detail with the current candidate context.
- LegacyPlateNesterAdapter: forward IProgress to the legacy engine so
  its progress surfaces under the active candidate.
- Tests: geometry (quadrants, rotations, touching, containment, holes,
  empty stock, real Default/Strip smoke), validation, and cancellation
  suites; repaired test fakes that emitted out-of-bounds or overlapping
  placements the gate now correctly rejects.

Engine.Tests: 70 passed, 0 failed, 0 skipped in Debug and Release.
Windows-only OpenNest.Tests not run on Linux.

OpenNest.Engine/Jobs/Adapters/LegacyPlateNesterAdapter.cs

@@ -46,7 +46,8 @@ public sealed class LegacyPlateNesterAdapter : IPlateNester
             });
         }
         var engine = engineFactory(plate) ?? throw new InvalidOperationException("Legacy engine factory returned null.");
-        var parts = engine.Nest(items, null, token);
+        var legacyProgress = progress == null ? null : new LegacyProgress(progress, request.Stock.Id);
+        var parts = engine.Nest(items, legacyProgress, token);
         token.ThrowIfCancellationRequested();
         if (parts == null) throw new InvalidOperationException("Legacy engine returned null placements.");
         var placements = new List<NestJobPlacement>();
@@ -58,4 +59,13 @@ public sealed class LegacyPlateNesterAdapter : IPlateNester
         }
         return new PlateCandidate(placements);
     }
+
+    private sealed class LegacyProgress(IProgress<NestJobProgress> progress, string stockId) : IProgress<NestProgress>
+    {
+        public void Report(NestProgress value)
+        {
+            ArgumentNullException.ThrowIfNull(value);
+            progress.Report(new NestJobProgress(NestJobStage.EvaluatingCandidate, stockId, -1, 0, 0, value));
+        }
+    }
 }