feat: Add Z Anti-Aliasing (ZAA) contouring support

Port Z Anti-Aliasing from BambuStudio-ZAA (https://github.com/adob/BambuStudio-ZAA)
to OrcaSlicer. ZAA eliminates stair-stepping on curved and sloped top surfaces
by raycasting each extrusion point against the original 3D mesh and micro-adjusting
Z height to follow the actual surface geometry.

Key changes:
- Add ContourZ.cpp raycasting algorithm (~330 lines)
- Extend geometry with 3D support (Point3, Line3, Polyline3, MultiPoint3)
- Template arc fitting for 2D/3D compatibility
- Change ExtrusionPath::polyline from Polyline to Polyline3
- Add 5 ZAA config options (zaa_enabled, zaa_min_z, etc.)
- Add posContouring pipeline step in PrintObject
- Update GCode writer for 3D coordinate output
- Add ZAA settings UI in Print Settings > Quality
- Add docs/ZAA.md with usage and implementation details

ZAA is opt-in and disabled by default. When disabled, the slicing pipeline
is unchanged.

src/libslic3r/Fill/Fill.cpp

@@ -1215,6 +1215,7 @@ void Layer::make_fills(FillAdaptive::Octree* adaptive_fill_octree, FillAdaptive:
         std::unique_ptr<Fill> f = std::unique_ptr<Fill>(Fill::new_from_type(surface_fill.params.pattern));
         f->set_bounding_box(bbox);
         f->layer_id = this->id();
+		f->dont_alternate_fill_direction = this->object()->config().zaa_dont_alternate_fill_direction;
         f->z 		= this->print_z;
         f->angle 	= surface_fill.params.angle;
         f->fixed_angle = surface_fill.params.fixed_angle;
@@ -1408,6 +1409,7 @@ Polylines Layer::generate_sparse_infill_polylines_for_anchoring(FillAdaptive::Oc
         std::unique_ptr<Fill> f = std::unique_ptr<Fill>(Fill::new_from_type(surface_fill.params.pattern));
         f->set_bounding_box(bbox);
         f->layer_id = this->id() - this->object()->get_layer(0)->id(); // We need to subtract raft layers.
+		f->dont_alternate_fill_direction = this->object()->config().zaa_dont_alternate_fill_direction;
         f->z        = this->print_z;
         f->angle    = surface_fill.params.angle;
         f->fixed_angle = surface_fill.params.fixed_angle;
@@ -1580,6 +1582,7 @@ void Layer::make_ironing()
     std::unique_ptr<Fill> f         = std::unique_ptr<Fill>(Fill::new_from_type(f_pattern));
     f->set_bounding_box(this->object()->bounding_box());
     f->layer_id = this->id();
+	f->dont_alternate_fill_direction = this->object()->config().zaa_dont_alternate_fill_direction;
     f->z        = this->print_z;
     f->overlap  = 0;
 	for (size_t i = 0; i < by_extruder.size();) {
@@ -1592,6 +1595,7 @@ void Layer::make_ironing()
             f = std::unique_ptr<Fill>(Fill::new_from_type(f_pattern));
             f->set_bounding_box(this->object()->bounding_box());
             f->layer_id = this->id();
+			f->dont_alternate_fill_direction = this->object()->config().zaa_dont_alternate_fill_direction;
             f->z        = this->print_z;
             f->overlap  = 0;
 		}

src/libslic3r/Fill/FillBase.cpp

@@ -308,7 +308,9 @@ std::pair<float, Point> Fill::_infill_direction(const Surface *surface) const
     } else if (this->layer_id != size_t(-1) && !fixed_angle) {
         // alternate fill direction
         //Orca: Do not alternate direction if Fill.fixed_angle is true
-        out_angle += this->_layer_angle(this->layer_id / surface->thickness_layers);
+        if (!this->dont_alternate_fill_direction) {
+            out_angle += this->_layer_angle(this->layer_id / surface->thickness_layers);
+        }
     } else {
 //    	printf("Layer_ID undefined!\n");
     }

src/libslic3r/Fill/FillBase.hpp

@@ -141,6 +141,7 @@ public:
 
     // BBS: all no overlap expolygons in same layer
     ExPolygons  no_overlap_expolygons;
+    bool dont_alternate_fill_direction = false;
 
     static float infill_anchor;
     static float infill_anchor_max;