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/BoundingBox.hpp

@@ -236,7 +236,18 @@ class BoundingBox3  : public BoundingBox3Base<Vec3crd>
 public:
     BoundingBox3() : BoundingBox3Base<Vec3crd>() {}
     BoundingBox3(const Vec3crd &pmin, const Vec3crd &pmax) : BoundingBox3Base<Vec3crd>(pmin, pmax) {}
-    BoundingBox3(const Points3& points) : BoundingBox3Base<Vec3crd>(points) {}
+    BoundingBox3(const Points3& points) : BoundingBox3Base<Vec3crd>() {
+        if (!points.empty()) {
+            this->min = points.front();
+            this->max = points.front();
+            for (const auto &p : points) {
+                this->min = this->min.cwiseMin(static_cast<const Vec3crd&>(p));
+                this->max = this->max.cwiseMax(static_cast<const Vec3crd&>(p));
+            }
+            this->defined = true;
+        }
+    }
+    BoundingBox3(const std::vector<Vec3crd>& points) : BoundingBox3Base<Vec3crd>(points) {}
 };
 
 class BoundingBoxf : public BoundingBoxBase<Vec2d>