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/slic3r/GUI/GUI_Factories.cpp

@@ -102,7 +102,9 @@ std::map<std::string, std::vector<SimpleSettingData>>  SettingsFactory::OBJECT_C
 
 std::map<std::string, std::vector<SimpleSettingData>>  SettingsFactory::PART_CATEGORY_SETTINGS=
 {
-    { L("Quality"), {{"ironing_type", "",8},{"ironing_flow", "",9},{"ironing_spacing", "",10},{"ironing_inset", "", 11},{"bridge_flow", "",11},{"make_overhang_printable", "",11},{"bridge_density", "", 1}
+    { L("Quality"), {{"ironing_type", "",8},{"ironing_flow", "",9},{"ironing_spacing", "",10},{"ironing_inset", "", 11},{"bridge_flow", "",11},{"make_overhang_printable", "",11},{"bridge_density", "", 1},
+                    {"ironing_expansion", "", 14},
+                    {"zaa_enabled", "", 1}, {"zaa_region_disable", "", 2}, {"zaa_minimize_perimeter_height", "", 3},  {"zaa_dont_alternate_fill_direction", "", 4}, {"zaa_min_z", "", 5}
                     }},
     { L("Strength"), {{"wall_loops", "",1},{"top_shell_layers", L("Top Solid Layers"),1},{"top_shell_thickness", L("Top Minimum Shell Thickness"),1},{"top_surface_density", L("Top Surface Density"),1},
                     {"bottom_shell_layers", L("Bottom Solid Layers"),1}, {"bottom_shell_thickness", L("Bottom Minimum Shell Thickness"),1},{"bottom_surface_density", L("Bottom Surface Density"),1},