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()); }