refactor(engine): move fill and strategy code to dedicated namespaces

Move fill algorithms to OpenNest.Engine.Fill namespace:
FillLinear, FillExtents, PairFiller, ShrinkFiller, Compactor,
RemnantFiller, RemnantFinder, FillScore, Pattern, PatternTiler,
PartBoundary, RotationAnalysis, AngleCandidateBuilder, and
AccumulatingProgress.

Move strategy layer to OpenNest.Engine.Strategies namespace:
IFillStrategy, FillContext, FillStrategyRegistry, FillHelpers,
and all built-in strategy implementations.

Add using directives to all consuming files across Engine, UI,
MCP, and Tests projects.

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

OpenNest.Engine/Fill/RemnantFiller.cs

@@ -1,9 +1,9 @@
+using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
 using System.Threading;
-using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Iteratively fills remnant boxes with items using a RemnantFinder.