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/Print.hpp

@@ -970,8 +970,21 @@ public:
     PrintObjectPtrs&            objects_mutable() { return m_objects; }
     PrintRegionPtrs&            print_regions_mutable() { return m_print_regions; }
     std::vector<size_t>         layers_sorted_for_object(float start, float end, std::vector<LayerPtrs> &layers_of_objects, std::vector<BoundingBox> &boundingBox_for_objects, VecOfPoints& objects_instances_shift);
+    struct SkirtBrimGroup {
+        struct Brim {
+            ExtrusionEntityCollection brim;
+            std::vector<ObjectID>     object_ids;
+        };
+
+        ExtrusionEntityCollection skirt;
+        std::vector<ObjectID>     object_ids;
+        // Brims stay separate unless Combine brims merges colliding brims inside this group.
+        std::vector<Brim>         brims;
+    };
+
     const ExtrusionEntityCollection& skirt() const { return m_skirt; }
-    const std::vector<ExtrusionEntityCollection>& skirt_groups() const { return m_skirt_groups; }
+    const std::vector<SkirtBrimGroup>& skirt_brim_groups() const { return m_skirt_brim_groups; }
+    bool has_shared_per_object_skirt() const { return m_has_shared_per_object_skirt; }
     // Convex hull of the 1st layer extrusions, for bed leveling and placing the initial purge line.
     // It encompasses the object extrusions, support extrusions, skirt, brim, wipe tower.
     // It does NOT encompass user extrusions generated by custom G-code,
@@ -1145,7 +1158,8 @@ private:
 
     // Ordered collections of extrusion paths to build skirt loops and brim.
     ExtrusionEntityCollection               m_skirt;
-    std::vector<ExtrusionEntityCollection>  m_skirt_groups;
+    std::vector<SkirtBrimGroup>             m_skirt_brim_groups;
+    bool                                    m_has_shared_per_object_skirt { false };
     // BBS: collecting extrusion paths to build brim by objs
     std::map<ObjectID, ExtrusionEntityCollection>         m_brimMap;
     std::map<ObjectID, ExtrusionEntityCollection>         m_supportBrimMap;