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/GCode/ConflictChecker.hpp

@@ -87,7 +87,7 @@ public:
         for (int i = b; i < e; ++i) {
             for (const ExtrusionPath &path : _piles[i].paths) {
                 if (path.is_force_no_extrusion() == false) {
-                    Polyline check_polyline = path.polyline;
+                    Polyline check_polyline = path.polyline.to_polyline();
                     check_polyline.translate(_offset);
                     Lines tmpLines = check_polyline.lines();
                     for (const Line &line : tmpLines) { lines.emplace_back(line, _id, path.role()); }

src/libslic3r/GCode/ExtrusionProcessor.hpp

@@ -81,7 +81,19 @@ std::vector<ExtendedPoint> estimate_points_properties(const POINTS
     if (input_points.empty())
         return {};
     float boundary_offset = PREV_LAYER_BOUNDARY_OFFSET ? 0.5 * flow_width : 0.0f;
-    auto  maybe_unscale   = [](const P &p) { return SCALED_INPUT ? unscaled(p) : p.template cast<double>(); };
+    auto  maybe_unscale   = [](const P &p) -> Vec2d {
+        if constexpr (P::RowsAtCompileTime == 3) {
+            // 3D point - extract XY only
+            if constexpr (SCALED_INPUT) {
+                return unscaled(p).template head<2>();
+            } else {
+                return p.template head<2>().template cast<double>();
+            }
+        } else {
+            // 2D point - use as is
+            return SCALED_INPUT ? unscaled(p) : p.template cast<double>();
+        }
+    };
 
     std::vector<ExtendedPoint> points;
     points.reserve(input_points.size() * (ADD_INTERSECTIONS ? 1.5 : 1));

src/libslic3r/GCode/PrintExtents.cpp

@@ -30,7 +30,7 @@ static inline BoundingBox extrusion_polyline_extents(const Polyline &polyline, c
 
 static inline BoundingBoxf extrusionentity_extents(const ExtrusionPath &extrusion_path)
 {
-    BoundingBox bbox = extrusion_polyline_extents(extrusion_path.polyline, coord_t(scale_(0.5 * extrusion_path.width)));
+    BoundingBox bbox = extrusion_polyline_extents(extrusion_path.polyline.to_polyline(), coord_t(scale_(0.5 * extrusion_path.width)));
     BoundingBoxf bboxf;
     if (! empty(bbox)) {
         bboxf.min = unscale(bbox.min);
@@ -44,7 +44,7 @@ static inline BoundingBoxf extrusionentity_extents(const ExtrusionLoop &extrusio
 {
     BoundingBox bbox;
     for (const ExtrusionPath &extrusion_path : extrusion_loop.paths)
-        bbox.merge(extrusion_polyline_extents(extrusion_path.polyline, coord_t(scale_(0.5 * extrusion_path.width))));
+        bbox.merge(extrusion_polyline_extents(extrusion_path.polyline.to_polyline(), coord_t(scale_(0.5 * extrusion_path.width))));
     BoundingBoxf bboxf;
     if (! empty(bbox)) {
         bboxf.min = unscale(bbox.min);
@@ -58,7 +58,7 @@ static inline BoundingBoxf extrusionentity_extents(const ExtrusionMultiPath &ext
 {
     BoundingBox bbox;
     for (const ExtrusionPath &extrusion_path : extrusion_multi_path.paths)
-        bbox.merge(extrusion_polyline_extents(extrusion_path.polyline, coord_t(scale_(0.5 * extrusion_path.width))));
+        bbox.merge(extrusion_polyline_extents(extrusion_path.polyline.to_polyline(), coord_t(scale_(0.5 * extrusion_path.width))));
     BoundingBoxf bboxf;
     if (! empty(bbox)) {
         bboxf.min = unscale(bbox.min);

src/libslic3r/GCode/SeamPlacer.cpp

@@ -1514,7 +1514,8 @@ void SeamPlacer::place_seam(const Layer *layer, ExtrusionLoop &loop,
       current.path_idx = next_idx_modulo(current.path_idx, loop.paths.size());
       current.segment_idx = 0;
     }
-    current.foot_pt = loop.paths[current.path_idx].polyline.points[current.segment_idx];
+    const Point3 &p3 = loop.paths[current.path_idx].polyline.points[current.segment_idx];
+    current.foot_pt = Point(p3.x(), p3.y());
     return current;
   };
 
@@ -1527,7 +1528,8 @@ void SeamPlacer::place_seam(const Layer *layer, ExtrusionLoop &loop,
   size_t closest_perimeter_point_index = 0;
   { // local space for the closest_perimeter_point_index
     Perimeter *closest_perimeter = nullptr;
-    ExtrusionLoop::ClosestPathPoint closest_point{0,0,loop.paths[0].polyline.points[0]};
+    const Point3 &init_p3 = loop.paths[0].polyline.points[0];
+    ExtrusionLoop::ClosestPathPoint closest_point{0,0,Point(init_p3.x(), init_p3.y())};
     size_t points_count = std::accumulate(loop.paths.begin(), loop.paths.end(), 0, [](size_t acc,const ExtrusionPath& p) {
       return acc + p.polyline.points.size();
     });