refactor: simplify CutOff methods and fix ActionCutOff issues

- Extract MakePoint/AxisBounds/CrossAxisBounds helpers in CutOff to
  eliminate repeated axis-dependent branching
- Simplify BuildProgram loop from 4 code paths to 2
- Use static EmptyExclusions to avoid per-part list allocations
- Fix double event subscription in ActionCutOff constructor
- Dispose debounce timer in DisconnectEvents
- Remove redundant BuildPerimeterCache call in OnMouseDown
- Extract TotalCutLength test helper, remove duplicate test

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

OpenNest/Actions/ActionCutOff.cs

@@ -23,14 +23,9 @@ namespace OpenNest.Actions
             : base(plateView)
         {
             settings = plateView.CutOffSettings;
-            perimeterCache = Plate.BuildPerimeterCache(plateView.Plate);
             debounceTimer = new Timer { Interval = 16 };
             debounceTimer.Tick += OnDebounce;
-
-            plateView.MouseMove += OnMouseMove;
-            plateView.MouseDown += OnMouseDown;
-            plateView.KeyDown += OnKeyDown;
-            plateView.Paint += OnPaint;
+            ConnectEvents();
         }
 
         public override void ConnectEvents()
@@ -46,6 +41,7 @@ namespace OpenNest.Actions
         public override void DisconnectEvents()
         {
             debounceTimer.Stop();
+            debounceTimer.Dispose();
             plateView.MouseMove -= OnMouseMove;
             plateView.MouseDown -= OnMouseDown;
             plateView.KeyDown -= OnKeyDown;
@@ -90,7 +86,6 @@ namespace OpenNest.Actions
 
             plateView.Plate.CutOffs.Add(cutoff);
             plateView.Plate.RegenerateCutOffs(settings);
-            perimeterCache = Plate.BuildPerimeterCache(plateView.Plate);
             plateView.Invalidate();
         }