fix(demos): DemoBuildMachineTool puts the spindle's pivot on the tool-mount offset

CreateFromChainCode now seeds S with a DynamicRotation about -Z through S's origin (the spindle
role); the sample moves the tool mount to (-72.40, 72.40, 176.44) right after, so it moves the
spindle's pivot there too -- otherwise turning the spindle would swing the tool around S's origin.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

MachineTool/DemoBuildMachineTool.cs

@@ -43,6 +43,9 @@ namespace Sample.MachineTool
                 "[O][Y][X][C][w];[O][Z][B][S][t]");
             if (dst.ToolMountTransformer is StaticTranslation st)
                 st.Trans = new vec3d_t(-72.40, 72.40, 176.44);
+            //the spindle turns the tool about its own axis: the rotation's pivot
+            //sits on the tool mount's origin, where the offset above moved it.
+            dst.SpindleTransformer.Pivot = new vec3d_t(-72.40, 72.40, 176.44);
             dst.TransformerB.Pivot = new vec3d_t(-72.4, -177.4, 225.94);
             Dictionary<string, Solid> solidMap = new Dictionary<string, Solid>()
             {