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/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));