Merge upstream/main into belt-printer

Brings belt-printer up to main 4b4a261787. Resolutions:

- G-code header (#15897, #15915): main moved the header, config and
  thumbnail block later in _do_export; write_belt_header() moves with it,
  still after the thumbnails and outside the BTT_TFT gate.
- _extrude: first-layer acceleration keeps the per-path first-layer plane
  test with main's cached nozzle index (#16028); main's set_speed out-param
  form (#16108) everywhere else.
- GCodeWriter (#16108): the arc-to-polyline fallback for machine mappings
  that cannot express G2/G3 now runs in the out-param extrude_arc_to_xy,
  which is the overload GCode calls, and appends to the caller's string.
- GCodeProcessorResult: the belt fields join main's forwarding assign.
- Clipper2 (#15969): belt arrange helpers take Slic3r::Point; the tree
  support join types lose their ClipperLib qualifier.
- CLI arrange (#15837): belt printers still reserve no wipe tower.
- Wipe tower options (#15841): the two new sparse-layer toggles are hidden
  for belt printers like the rest of the tower options.
- Keyboard shortcuts (#15706): main's registry replaces the old key switch;
  the belt view toggle is re-registered in the next commit.
- Print::process: the belt purge-plan undo runs before main's SliceStarted
  event.
- scripts/filament_id_snapshot.json: deleted on main (a77209af8f).
- Includes and appended tests: union of both sides.

src/libslic3r/PrintObjectSlice.cpp

@@ -1,20 +1,46 @@
+#include <algorithm>
 #include <boost/log/trivial.hpp>
 #include <limits>
 
+#include <cstddef>
+#include <functional>
+#include <cassert>
+#include <cstdlib>
+#include <memory>
+#include <math.h>
+#include <cmath>
+#include <iterator>
 #include <tbb/parallel_for.h>
+#include <vector>
+#include <utility>
 
+#include "BoundingBox.hpp"
 #include "ClipperUtils.hpp"
 #include "ElephantFootCompensation.hpp"
+#include "ExPolygon.hpp"
 #include "Exception.hpp"
+#include "Flow.hpp"
 #include "I18N.hpp"
 #include "Layer.hpp"
+#include "Model.hpp"
 #include "MultiMaterialSegmentation.hpp"
+#include "ObjectID.hpp"
+#include "Point.hpp"
+#include "Polygon.hpp"
 #include "Print.hpp"
 #include "BeltTransform.hpp"
 #include "BeltSliceStrategy.hpp"
 #include "Geometry.hpp"
 //BBS
+#include "PrintConfig.hpp"
+#include "PrintBase.hpp"
 #include "ShortestPath.hpp"
+#include "libslic3r.h"
+#include "TriangleMeshSlicer.hpp"
+#include "TriangleMesh.hpp"
+#include "Slicing.hpp"
+#include "Surface.hpp"
+#include "Utils.hpp"
 #include "libslic3r/Feature/Interlocking/InterlockingGenerator.hpp"
 
 //! macro used to mark string used at localization, return same string
@@ -1802,4 +1828,16 @@ std::vector<Polygons> PrintObject::slice_support_volumes(const ModelVolumeType m
     return slices;
 }
 
+std::vector<ExPolygons> PrintObject::slice_single_volume_regions(const ModelVolume* volume) const
+{
+    if (volume == nullptr)
+        return {};
+    // Match the existing slicing heights and centered transform without flattening holes.
+    const std::vector<float> zs = zs_from_layers(this->layers());
+    MeshSlicingParamsEx params;
+    params.trafo = this->trafo_centered();
+    const Print *print = this->print();
+    return slice_volume(*volume, zs, params, [print]() { print->throw_if_canceled(); });
+}
+
 } // namespace Slic3r