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/Preset.cpp

@@ -953,6 +953,8 @@ static std::vector<std::string> s_Preset_print_options {
      "enable_wrapping_detection",
      "seam_slope_type", "seam_slope_conditional", "scarf_angle_threshold", "scarf_joint_speed", "scarf_joint_flow_ratio", "seam_slope_start_height", "seam_slope_entire_loop", "seam_slope_min_length", "seam_slope_steps", "seam_slope_inner_walls", "scarf_overhang_threshold",
      "interlocking_beam", "interlocking_orientation", "interlocking_beam_layer_count", "interlocking_depth", "interlocking_boundary_avoidance", "interlocking_beam_width","calib_flowrate_topinfill_special_order",
+     // Z Anti-Aliasing (ZAA)
+     "zaa_enabled", "zaa_minimize_perimeter_height", "zaa_dont_alternate_fill_direction", "zaa_min_z", "zaa_region_disable", "ironing_expansion",
 };
 
 static std::vector<std::string> s_Preset_filament_options {/*"filament_colour", */ "default_filament_colour", "required_nozzle_HRC", "filament_diameter", "pellet_flow_coefficient", "volumetric_speed_coefficients", "filament_type",