refactor: standardize fill strategy progress reporting via FillContext

Strategies and fillers previously called NestEngineBase.ReportProgress
directly, each constructing ProgressReport structs with phase, plate
number, and work area manually. Some strategies (RectBestFit) reported
nothing at all. This made progress updates inconsistent and flakey.

Add FillContext.ReportProgress(parts, description) as the single
standard method for intermediate progress. RunPipeline sets ActivePhase
before each strategy, and the context handles common fields. Lower-level
fillers (PairFiller, FillExtents, StripeFiller) now accept an
Action<List<Part>, string> callback instead of raw IProgress, removing
their coupling to NestEngineBase and ProgressReport.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/Fill/PairFiller.cs

@@ -45,9 +45,8 @@ namespace OpenNest.Engine.Fill
         }
 
         public PairFillResult Fill(NestItem item, Box workArea,
-            int plateNumber = 0,
             CancellationToken token = default,
-            IProgress<NestProgress> progress = null)
+            Action<List<Part>, string> reportProgress = null)
         {
             var bestFits = BestFitCache.GetOrCompute(
                 item.Drawing, plateSize.Length, plateSize.Width, partSpacing);
@@ -58,7 +57,7 @@ namespace OpenNest.Engine.Fill
 
             var targetCount = item.Quantity > 0 ? item.Quantity : 0;
             var parts = EvaluateCandidates(candidates, item.Drawing, workArea, targetCount,
-                plateNumber, token, progress);
+                token, reportProgress);
 
             return new PairFillResult { Parts = parts, BestFits = bestFits };
         }
@@ -66,7 +65,7 @@ namespace OpenNest.Engine.Fill
         private List<Part> EvaluateCandidates(
             List<BestFitResult> candidates, Drawing drawing,
             Box workArea, int targetCount,
-            int plateNumber, CancellationToken token, IProgress<NestProgress> progress)
+            CancellationToken token, Action<List<Part>, string> reportProgress)
         {
             List<Part> best = null;
             var sinceImproved = 0;
@@ -112,14 +111,8 @@ namespace OpenNest.Engine.Fill
                             sinceImproved++;
                         }
 
-                        NestEngineBase.ReportProgress(progress, new ProgressReport
-                        {
-                            Phase = NestPhase.Pairs,
-                            PlateNumber = plateNumber,
-                            Parts = best,
-                            WorkArea = workArea,
-                            Description = $"Pairs: {batchStart + j + 1}/{candidates.Count} candidates, best = {best?.Count ?? 0} parts",
-                        });
+                        reportProgress?.Invoke(best,
+                            $"Pairs: {batchStart + j + 1}/{candidates.Count} candidates, best = {best?.Count ?? 0} parts");
                     }
 
                     if (batchEnd >= EarlyExitMinTried && sinceImproved >= EarlyExitStaleLimit)