fix(engine): fix pair candidate filtering for narrow plates and strips

The BestFitFilter's aspect ratio cap of 5.0 was rejecting valid pair
candidates needed for narrow plates (e.g. 60x6.5, aspect 9.2) and
remainder strips on normal plates. Three fixes:

- BestFitFinder: derive MaxAspectRatio from the plate's own aspect
  ratio so narrow plates don't reject all elongated pairs
- SelectPairCandidates: search the full unfiltered candidate list
  (not just Keep=true) in strip mode, so pairs rejected by aspect
  ratio for the main plate can still be used for narrow remainder
  strips
- BestFitCache.Populate: skip caching empty result lists so stale
  pre-computed data from nest files doesn't prevent recomputation

Also fixes console --size parsing to use LxW format matching
Size.Parse convention, and includes prior engine refactoring
(sequential fill loops, parallel FillPattern, pre-sorted edge
arrays in RotationSlideStrategy).

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

OpenNest.Core/Helper.cs

@@ -1103,7 +1103,7 @@ namespace OpenNest
             return minDist;
         }
 
-        private static double OneWayDistance(
+        public static double OneWayDistance(
             Vector vertex, (Vector start, Vector end)[] edges, Vector edgeOffset,
             PushDirection direction)
         {