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.

OpenNest.Engine.Tests/Jobs/NestJobExampleTests.cs

@@ -145,7 +145,7 @@ public class NestJobExampleTests
                     quadrant: 1
                 ),
             },
-            new NestJobOptions("Default", maxPlates: 1)
+            new NestJobOptions("Fill", maxPlates: 1)
         );
 
         var result = new NestJobRunner(PlateNesterFactory.Create).Solve(job);
@@ -172,7 +172,7 @@ public class NestJobExampleTests
         var item = new NestItem { Drawing = drawing, Quantity = 1 };
 
         var parts = PlateFillService.Nest(
-            "Default",
+            "Fill",
             plate,
             new List<NestItem> { item },
             progress: null,