Reset the Fan Mover Instead of Releasing It (#16386)

Clearing per-export state called unique_ptr::release(), which gives up ownership without deleting. Every export uses a new GCode, so the pointer is still empty there today and nothing leaks; reset() states the intent and would free the object if a GCode were ever reused.

src/libslic3r/GCode.cpp

@@ -3295,7 +3295,7 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
     m_is_role_based_fan_on.fill(false);
     m_role_based_fan_marker_layer.fill(-1);
 
-    m_fan_mover.release();
+    m_fan_mover.reset();
     m_ordering_cache.clear();
 
     // Belt printer: initialize belt-specific writer via virtual hook.