Merge upstream/main into iXex PR branch (#13086)

Catches the iXex/IDEX parallel-printing branch up to upstream main
(102 commits). Two content conflicts resolved:

- src/libslic3r/Preset.cpp: s_Preset_printer_options — kept upstream's
  new "use_3mf" key and our iMEX printer-capability/mode keys.
- tests/fff_print/test_gcodewriter.cpp: upstream revived the disabled
  suite (#14196), dropping the obsolete [.]-tagged lift() test and its
  config_lift_unlift.ini; kept their set_speed + z_hop tests and appended
  our 10 per-firmware set_pressure_advance/set_temperature scenarios.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/libslic3r/GCode.hpp

@@ -352,7 +352,16 @@ private:
         const float skirt_start_angle,
         const LayerTools &layer_tools,
         const Layer& layer,
+        unsigned int extruder_id,
+        std::vector<coordf_t> &skirt_done);
+    std::string generate_object_skirt_group(const Print &print,
+        const PrintObject &object,
+        const LayerTools &layer_tools,
+        const Layer& layer,
         unsigned int extruder_id);
+    std::string generate_object_brim(const Print &print,
+        const PrintObject &object,
+        bool first_layer);
 
     LayerResult process_layer(
         const Print                     &print,
@@ -626,8 +635,8 @@ private:
 
     std::unique_ptr<SmallAreaInfillFlowCompensator> m_small_area_infill_flow_compensator;
     
-    // Heights (print_z) at which the skirt has already been extruded.
-    std::vector<coordf_t>               m_skirt_done;
+    // Heights (print_z) at which each grouped skirt has already been extruded.
+    std::vector<std::vector<coordf_t>>   m_skirt_group_done;
     // Has the brim been extruded already? Brim is being extruded only for the first object of a multi-object print.
     bool                                m_brim_done;
     // Flag indicating whether the nozzle temperature changes from 1st to 2nd layer were performed.