feat: add Disable/Enable API to FillStrategyRegistry

Adds methods to permanently disable/enable strategies by name. Disabled
strategies remain registered but are excluded from the default pipeline.
SetEnabled (used for remnant fills) takes precedence over the disabled
set, so explicit overrides still work.

Pipeline test now checks against active strategy count dynamically.

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

OpenNest.Tests/Strategies/FillPipelineTests.cs

@@ -1,3 +1,4 @@
+using OpenNest.Engine.Strategies;
 using OpenNest.Geometry;
 
 namespace OpenNest.Tests.Strategies;
@@ -24,8 +25,8 @@ public class FillPipelineTests
 
         engine.Fill(item, plate.WorkArea(), null, System.Threading.CancellationToken.None);
 
-        Assert.True(engine.PhaseResults.Count >= 6,
-            $"Expected phase results from all strategies, got {engine.PhaseResults.Count}");
+        Assert.True(engine.PhaseResults.Count >= FillStrategyRegistry.Strategies.Count,
+            $"Expected phase results from all active strategies, got {engine.PhaseResults.Count}");
     }
 
     [Fact]