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.Tests/Jobs/FiniteStockJobTests.cs

@@ -16,7 +16,7 @@ public class FiniteStockJobTests
     }
 
     internal static PlateCandidate One(PlatePlacementRequest request) => new(new[]
-        { new NestJobPlacement(request.Parts[0].Id, 99, 1, 2, 0) });
+        { new NestJobPlacement(request.Parts[0].Id, 99, 0, 0, 0) });
 
     [Theory]
     [InlineData(3, 3, 0, NestJobStatus.Complete, NestJobStopReason.Completed)]
@@ -108,7 +108,7 @@ public class FiniteStockJobTests
     public void MixedStockCanBeEvaluated()
     {
         var job = Job();
-        var mixed = new NestJob(job.Parts, job.Plates.Concat(new[] { new NestPlateStock("other", new Size(10, 20), 2) }));
+        var mixed = new NestJob(job.Parts, job.Plates.Concat(new[] { new NestPlateStock("other", new Size(20, 10), 2) }));
         var result = new NestJobRunner(_ => new Nester(One)).Solve(mixed);
         Assert.Equal(NestJobStatus.Complete, result.Status);
     }