refactor(engine): introduce PairFillResult and remove FillRemainingStrip

PairFiller now returns PairFillResult (Parts + BestFits) instead of
using a mutable BestFits property. Extracted EvaluateCandidates,
TryReduceWorkArea, and BuildTilingAngles for clarity. Simplified the
candidate loop by leveraging FillScore comparison semantics.

Removed FillRemainingStrip and all its helpers (FindPlacedEdge,
BuildRemainingStrip, BuildRotationSet, FindBestFill, TryFewerRows,
RemainderPatterns) from FillLinear — these were a major bottleneck in
strip nesting, running expensive fills on undersized remnant strips.
ShrinkFiller + RemnantFiller already handle space optimization, making
the remainder strip fill redundant.

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

OpenNest.Engine/Strategies/PairsFillStrategy.cs

@@ -15,9 +15,9 @@ namespace OpenNest.Engine.Strategies
             var result = filler.Fill(context.Item, context.WorkArea,
                 context.PlateNumber, context.Token, context.Progress);
 
-            context.SharedState["BestFits"] = filler.BestFits;
+            context.SharedState["BestFits"] = result.BestFits;
 
-            return result;
+            return result.Parts;
         }
     }
 }