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/DefaultNestEngine.cs

@@ -295,6 +295,7 @@ namespace OpenNest
                 foreach (var strategy in FillStrategyRegistry.Strategies)
                 {
                     context.Token.ThrowIfCancellationRequested();
+                    context.ActivePhase = strategy.Phase;
 
                     var sw = Stopwatch.StartNew();
                     var result = strategy.Fill(context);