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/NestJobProgress.cs

@@ -0,0 +1,10 @@
+namespace OpenNest;
+
+public enum NestJobStage { EvaluatingCandidate, PlateCommitted }
+
+/// <summary>
+/// Whole-job progress. Counts change only after a physical sheet commits; LegacyProgress is optional
+/// non-authoritative detail from a plate nester while its candidate remains under evaluation.
+/// </summary>
+public sealed record NestJobProgress(NestJobStage Stage, string StockId, int PlateIndex,
+    int CommittedPlates, int CommittedParts, NestProgress LegacyProgress = null);