feat(engine): make Default choose between Irregular and Rectangles per job

Default is the engine every front end uses when none is named. It now runs
Irregular, then Rectangles, checks both layouts with NestLayoutCheck and keeps
the best: valid first, then fewest unplaced parts, then lowest salvage-credited
cost; ties keep Irregular. A candidate that throws or returns nothing is
skipped, cancellation stops the search, and only the chosen layout's plate
commits are reported. Neither engine wins every job in the lane benchmarks,
and Rectangles adds little time while also covering Irregular's invalid
layouts.

The registry lists Default first and no longer maps the name to Fill; fill
strategy callers still read Default as Fill. A future circle/ring engine joins
as another candidate.

scripts/ReleaseSmoke/Program.cs

@@ -20,7 +20,7 @@ try
     var create = registry.GetMethod("Create")!;
 
     // Every built-in engine the desktop offers must instantiate from the packaged assembly.
-    string[] expected = ["Rectangles", "Irregular", "StockLadder", "Fill", "Strip", "Vertical Remnant", "Horizontal Remnant"];
+    string[] expected = ["Default", "Rectangles", "Irregular", "StockLadder", "Fill", "Strip", "Vertical Remnant", "Horizontal Remnant"];
     foreach (var name in expected)
     {
         var engine = create.Invoke(null, [name])!;