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/ClipperUtils.cpp

@@ -411,6 +411,19 @@ Slic3r::ExPolygons offset_ex(const Slic3r::Polygons &polygons, const float delta
 
 Slic3r::Polygons offset(const Slic3r::Polyline &polyline, const float delta, ClipperLib::JoinType joinType, double miterLimit, ClipperLib::EndType end_type)
     { assert(delta > 0); return to_polygons(clipper_union<ClipperLib::Paths>(raw_offset_polyline(ClipperUtils::SinglePathProvider(polyline.points), delta, joinType, miterLimit, end_type))); }
+
+Slic3r::Polygons offset(const Slic3r::Polyline3 &polyline, const float delta, ClipperLib::JoinType joinType, double miterLimit, ClipperLib::EndType end_type)
+{
+    assert(delta > 0);
+    return to_polygons(
+        clipper_union<ClipperLib::Paths>(
+            raw_offset_polyline(
+                ClipperUtils::SinglePathProvider(polyline.to_polyline().points),
+                delta,
+                joinType,
+                miterLimit,
+                end_type)));
+}
 Slic3r::Polygons offset(const Slic3r::Polylines &polylines, const float delta, ClipperLib::JoinType joinType, double miterLimit, ClipperLib::EndType end_type)
     { assert(delta > 0); return to_polygons(clipper_union<ClipperLib::Paths>(raw_offset_polyline(ClipperUtils::PolylinesProvider(polylines), delta, joinType, miterLimit, end_type))); }