refactor(engine): rename the Default engine and fill strategy to Fill

The multi-phase lattice fill (linear, pairs, rectangle best-fit, remainder) is
no longer meant to be the engine used by default, so it gets a name for what
it does. The registry lists it as Fill and maps the old name Default to it;
PlateFillService, PlateNesterFactory and NestJobOptions use Fill, and every
fill-strategy caller still accepts Default. Console --autonest now validates
engine names through NestingEngineRegistry.ResolveName so renamed names work.

Layouts are unchanged: Default and Fill resolve to the same fillers and the
golden layouts pass under the new name.

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", "Default", "Strip", "Vertical Remnant", "Horizontal Remnant"];
+    string[] expected = ["Rectangles", "Irregular", "StockLadder", "Fill", "Strip", "Vertical Remnant", "Horizontal Remnant"];
     foreach (var name in expected)
     {
         var engine = create.Invoke(null, [name])!;