docs(engines): template on shared services and a determinism rule

New engines start from the shared test kit (the template's tests are a
one-line EngineContractTests subclass) and the host APIs, so they don't
re-derive geometry reading, work areas or validator tolerances.
BENCH-RULES.md now forbids clocks, unseeded randomness and environment
variables from influencing placement (budgets count work; wall time only
through the host's cancellation token) and lists the kit as read-only.
Build-Engines.ps1 deploys only OpenNest.Engine.* folders, and
New-Engine.ps1 -IncludeBuildFiles copies the kit.

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

_Template/__NAME__NestingEngine.cs

@@ -18,6 +18,7 @@ public sealed class __NAME__NestingEngine : INestingEngine
     )
     {
         ArgumentNullException.ThrowIfNull(job);
+        token.ThrowIfCancellationRequested();
 
         // TODO: implement independent placement logic here.
         //
@@ -30,9 +31,12 @@ public sealed class __NAME__NestingEngine : INestingEngine
         // job.Plates   -> candidate stock sheets (size, spacing, edge spacing, quadrant, quantity)
         // job.Options  -> job-wide options
         //
-        // Return a NestJobResult built from NestJobPlateResult (one per used sheet, holding
-        // ordered NestJobPlacement values), PartFulfillment (requested vs placed per part id),
-        // and StockUsage (sheets used per stock id).
+        // Read material with JobPartGeometry.TryRead(part.Geometry); use stock.WorkArea/Fits,
+        // RotationCandidates.ForShape and NestJobCost as primitives for your own decisions.
+        // var result = new NestJobResultBuilder(job, progress);
+        // result.AddSheet(stock, poses); // (PartId, X, Y, Rotation); call only on commit.
+        // return result.Build(NestJobStopReason.NoPlacementFound);
+        // The builder assigns sheet/instance indices, fulfillment, usage and commit progress.
 
         throw new NotImplementedException("__NAME__ nesting engine placement logic not yet implemented.");
     }