Refactor skirt/brim + bugfixes related to them (#14333)

Refactor skirt and brim ownership and emission flow

Refactor skirt and brim generation around a common object/group
ownership model.

Skirts and brims are now emitted as a coordinated preamble
(skirt -> brim -> object) instead of being generated and emitted
through multiple independent code paths.

Changes:
- Fix repeated skirt emission caused by the previous skirt state
  tracking logic.
- Restore local skirt/brim ordering for per-object skirts in
  By Layer mode.
- Emit brims together with their owning object or object group.
- Handle combined brims independently from skirt grouping.
- Handle draft shields through the same ownership model as skirts.
- Fix draft shield generation when skirt height is zero.
- Generate draft shields after brim geometry is known, preventing
  draft shields from overlapping brims.
- Reject unsafe grouped per-object skirt configurations in
  By Object mode.
- Remove legacy skirt emission paths and state-management
  workarounds.

Support brim generation remains unchanged.

Co-authored-by: SoftFever <softfeverever@gmail.com>

src/libslic3r/GCode.hpp

@@ -343,7 +343,16 @@ 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,
@@ -609,8 +618,8 @@ 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.