perf: parallelize PairFiller candidates and add GridDedup

- Evaluate pair candidates in parallel batches instead of sequentially
- Add GridDedup to skip duplicate pattern/direction/workArea combos
  across PairFiller and StripeFiller strategies
- Replace crude 30% remnant area estimate with L-shaped geometry
  calculation using actual grid extents and max utilization
- Move FillStrategyRegistry.SetEnabled to outer evaluation loop
  to avoid repeated enable/disable per remnant fill

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

OpenNest.Engine/Fill/GridDedup.cs

@@ -0,0 +1,75 @@
+using System;
+using System.Collections.Concurrent;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Fill;
+
+/// <summary>
+/// Tracks evaluated grid configurations so duplicate pattern/direction/workArea
+/// combinations can be skipped across fill strategies.
+/// </summary>
+public class GridDedup
+{
+    public const string SharedStateKey = "GridDedup";
+
+    private readonly ConcurrentDictionary<GridKey, byte> _seen = new();
+
+    /// <summary>
+    /// Returns true if this configuration has NOT been seen before (i.e., should be evaluated).
+    /// Returns false if it's a duplicate.
+    /// </summary>
+    public bool TryAdd(Box patternBox, Box workArea, NestDirection dir)
+    {
+        var key = new GridKey(patternBox, workArea, dir);
+        return _seen.TryAdd(key, 0);
+    }
+
+    public int Count => _seen.Count;
+
+    /// <summary>
+    /// Gets or creates a GridDedup from FillContext.SharedState.
+    /// </summary>
+    public static GridDedup GetOrCreate(System.Collections.Generic.Dictionary<string, object> sharedState)
+    {
+        if (sharedState.TryGetValue(SharedStateKey, out var existing))
+            return (GridDedup)existing;
+
+        var dedup = new GridDedup();
+        sharedState[SharedStateKey] = dedup;
+        return dedup;
+    }
+
+    private readonly struct GridKey : IEquatable<GridKey>
+    {
+        private readonly int _patternW, _patternL, _workW, _workL, _dir;
+
+        public GridKey(Box patternBox, Box workArea, NestDirection dir)
+        {
+            _patternW = (int)System.Math.Round(patternBox.Width * 10);
+            _patternL = (int)System.Math.Round(patternBox.Length * 10);
+            _workW = (int)System.Math.Round(workArea.Width * 10);
+            _workL = (int)System.Math.Round(workArea.Length * 10);
+            _dir = (int)dir;
+        }
+
+        public bool Equals(GridKey other) =>
+            _patternW == other._patternW && _patternL == other._patternL &&
+            _workW == other._workW && _workL == other._workL &&
+            _dir == other._dir;
+
+        public override bool Equals(object obj) => obj is GridKey other && Equals(other);
+
+        public override int GetHashCode()
+        {
+            unchecked
+            {
+                var hash = _patternW;
+                hash = hash * 397 ^ _patternL;
+                hash = hash * 397 ^ _workW;
+                hash = hash * 397 ^ _workL;
+                hash = hash * 397 ^ _dir;
+                return hash;
+            }
+        }
+    }
+}