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/FixedStrategyNestingEngineTests.cs

@@ -22,7 +22,7 @@ public class FixedStrategyNestingEngineTests
     [Fact]
     public void PreservesJobMaxPlates()
     {
-        var engine = new FixedStrategyNestingEngine("Default");
+        var engine = new FixedStrategyNestingEngine("Fill");
         var part = new NestJobPart(
             "part",
             PartGeometrySnapshot.FromProgram(TestDrawingFactory.Rectangle(100, 100)),
@@ -39,7 +39,7 @@ public class FixedStrategyNestingEngineTests
         var job = new NestJob(
             new[] { part },
             new[] { stock },
-            new NestJobOptions("Default", maxPlates: 1)
+            new NestJobOptions("Fill", maxPlates: 1)
         );
 
         var result = engine.Solve(job);