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/Support/SupportCommon.cpp

@@ -1163,7 +1163,7 @@ static void modulate_extrusion_by_overlapping_layers(
         for (ExtrusionEntity *ee : extrusions_in_out) {
             ExtrusionPath *path = dynamic_cast<ExtrusionPath*>(ee);
             assert(path != nullptr);
-            polylines.emplace_back(Polyline(std::move(path->polyline)));
+            polylines.emplace_back(path->polyline.to_polyline());
             path_ends.emplace_back(std::pair<Point, Point>(polylines.back().points.front(), polylines.back().points.back()));
             delete path;
         }
@@ -1288,9 +1288,10 @@ static void modulate_extrusion_by_overlapping_layers(
             if (! path->polyline.points.empty())
                 path->polyline.points.pop_back();
             // Consume the fragment's polyline, remove it from the input fragments, so it will be ignored the next time.
-            path->polyline.append(std::move(frag_polyline));
+            path->polyline.append(Polyline3(std::move(frag_polyline)));
             frag_polyline.points.clear();
-            pt_current = path->polyline.points.back();
+            const Point3 &pt_back3 = path->polyline.points.back();
+            pt_current = Point(pt_back3.x(), pt_back3.y());
             if (pt_current == pt_end) {
                 // End of the path.
                 break;