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/PrintObjectSlice.cpp

@@ -4,6 +4,7 @@
 
 #include "ClipperUtils.hpp"
 #include "ElephantFootCompensation.hpp"
+#include "Exception.hpp"
 #include "I18N.hpp"
 #include "Layer.hpp"
 #include "MultiMaterialSegmentation.hpp"
@@ -34,6 +35,13 @@ LayerPtrs new_layers(
         coordf_t lo = object_layers[i_layer];
         coordf_t hi = object_layers[i_layer + 1];
         coordf_t slice_z = 0.5 * (lo + hi);
+        if (print_object->config().zaa_enabled) {
+            coordf_t z_offset = print_object->config().zaa_min_z;
+            slice_z = lo + z_offset;
+            if (slice_z < lo || slice_z > hi) {
+                throw RuntimeError("Bad min Z value");
+            }
+        }
         Layer *layer = new Layer(id ++, print_object, hi - lo, hi + zmin, slice_z);
         out.emplace_back(layer);
         if (prev != nullptr) {