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/slic3r/GUI/3DScene.cpp

@@ -1991,7 +1991,7 @@ void _3DScene::thick_lines_to_verts(
 // Fill in the qverts and tverts with quads and triangles for the extrusion_path.
 void _3DScene::extrusionentity_to_verts(const ExtrusionPath& extrusion_path, float print_z, const Point& copy, GUI::GLModel::Geometry& geometry)
 {
-    Polyline            polyline = extrusion_path.polyline;
+    Polyline            polyline = extrusion_path.polyline.to_polyline();
     polyline.remove_duplicate_points();
     polyline.translate(copy);
     const Lines               lines = polyline.lines();
@@ -2007,7 +2007,7 @@ void _3DScene::extrusionentity_to_verts(const ExtrusionLoop& extrusion_loop, flo
     std::vector<double> widths;
     std::vector<double> heights;
     for (const ExtrusionPath& extrusion_path : extrusion_loop.paths) {
-        Polyline            polyline = extrusion_path.polyline;
+        Polyline            polyline = extrusion_path.polyline.to_polyline();
         polyline.remove_duplicate_points();
         polyline.translate(copy);
         const Lines lines_this = polyline.lines();
@@ -2025,7 +2025,7 @@ void _3DScene::extrusionentity_to_verts(const ExtrusionMultiPath& extrusion_mult
     std::vector<double> widths;
     std::vector<double> heights;
     for (const ExtrusionPath& extrusion_path : extrusion_multi_path.paths) {
-        Polyline            polyline = extrusion_path.polyline;
+        Polyline            polyline = extrusion_path.polyline.to_polyline();
         polyline.remove_duplicate_points();
         polyline.translate(copy);
         const Lines lines_this = polyline.lines();