Belt G-code: reset the first-layer band state per object

In by-object printing the cooling buffer is reset for every object, but
m_belt_in_band, which tracks whether the extrusion is inside the band
along the belt where the part fan stays off, kept the previous object's
value.  If the previous object ended inside the band the next one never
emitted its band start marker.  Reset it with the cooling buffer.

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

src/libslic3r/GCode.cpp

@@ -4176,6 +4176,9 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
                 // Reset the cooling buffer internal state (the current position, feed rate, accelerations).
                 m_cooling_buffer->set_current_extruder(initial_extruder_id, get_extruder_id(initial_extruder_id));
                 m_cooling_buffer->reset(this->writer().get_position());
+                // The belt first-layer band is tracked per object as well: if the previous
+                // object ended inside the band, this one has to open its own.
+                m_belt_in_band = false;
                 // Process all layers of a single object instance (sequential mode) with a parallel pipeline:
                 // Generate G-code, run the filters (vase mode, cooling buffer), run the G-code analyser
                 // and export G-code into file.