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/SupportMaterial.cpp

@@ -1181,7 +1181,9 @@ namespace SupportMaterialInternal {
                         // This is a complete loop.
                         // Add the outer contour first.
                         Polygon poly;
-                        poly.points = ep.polyline.points;
+                        // Convert Points3 to Points
+                        for (const Point3 &p3 : ep.polyline.points)
+                            poly.points.emplace_back(p3.x(), p3.y());
                         poly.points.pop_back();
                         if (poly.area() < 0)
                             poly.reverse();