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/Strategies/FillHelpers.cs

@@ -113,13 +113,12 @@ namespace OpenNest.Engine.Strategies
         /// <summary>
         /// Sweeps a list of angles, calling fillAtAngle for each, and returns
         /// the best result according to the context's comparer. Handles
-        /// cancellation and progress reporting.
+        /// cancellation and progress reporting via context.ReportProgress.
         /// </summary>
         public static List<Part> BestOverAngles(
             FillContext context,
             IReadOnlyList<double> angles,
             Func<double, List<Part>> fillAtAngle,
-            NestPhase phase,
             string phaseLabel)
         {
             var workArea = context.WorkArea;
@@ -140,14 +139,8 @@ namespace OpenNest.Engine.Strategies
                         best = result;
                 }
 
-                NestEngineBase.ReportProgress(context.Progress, new ProgressReport
-                {
-                    Phase = phase,
-                    PlateNumber = context.PlateNumber,
-                    Parts = best,
-                    WorkArea = workArea,
-                    Description = $"{phaseLabel}: {i + 1}/{angles.Count} angles, {angleDeg:F0}° best = {best?.Count ?? 0} parts",
-                });
+                context.ReportProgress(best,
+                    $"{phaseLabel}: {i + 1}/{angles.Count} angles, {angleDeg:F0}° best = {best?.Count ?? 0} parts");
             }
 
             return best ?? new List<Part>();