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.

tests/fff_print/test_helpers.cpp

@@ -1,5 +1,6 @@
 #include "test_helpers.hpp"
 
+#include <catch2/catch_test_macros.hpp>
 #include "libslic3r/TriangleMesh.hpp"
 #include "libslic3r/GCodeReader.hpp"
 #include "libslic3r/Config.hpp"
@@ -7,11 +8,23 @@
 #include "libslic3r/Format/OBJ.hpp"
 #include "libslic3r/Format/STL.hpp"
 
+#include <algorithm>
 #include <cstdlib>
+#include "libslic3r/libslic3r.h"
+#include "libslic3r/Exception.hpp"
+#include "libslic3r/PrintConfig.hpp"
+#include <initializer_list>
+#include "libslic3r/Point.hpp"
+#include <fstream>
+#include <iterator>
+#include <set>
 #include <string>
 
 #include <boost/filesystem.hpp>
 #include <libslic3r/ModelArrange.hpp>
+#include <unordered_map>
+#include <utility>
+#include <vector>
 
 #include "test_utils.hpp"
 
@@ -225,7 +238,7 @@ DynamicPrintConfig multifilament_config(unsigned int filaments, std::initializer
 }
 
 void init_print(std::vector<TriangleMesh> &&meshes, Slic3r::Print &print, Slic3r::Model &model, const DynamicPrintConfig &config_in,
-                const std::vector<std::vector<ConfigBase::SetDeserializeItem>> *per_object_overrides, bool arrange)
+                const std::vector<std::vector<ConfigBase::SetDeserializeItem>> *per_object_overrides, bool arrange, size_t instances)
 {
 	DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
     config.apply(config_in);
@@ -236,7 +249,8 @@ void init_print(std::vector<TriangleMesh> &&meshes, Slic3r::Print &print, Slic3r
 		ModelObject *object = model.add_object();
 		object->name += "object.stl";
 		object->add_volume(std::move(t));
-		object->add_instance();
+		for (size_t i = 0; i < instances; ++i)
+			object->add_instance();
 
 		if (per_object_overrides && object_idx < per_object_overrides->size() && !(*per_object_overrides)[object_idx].empty()) {
 			DynamicPrintConfig oc;