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/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");
     }