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/PrintConfig.hpp

@@ -1027,6 +1027,11 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionInt,  interlocking_depth))
     ((ConfigOptionInt,  interlocking_boundary_avoidance))
 
+    // Z Anti-Aliasing (aka Z Contouring)
+    ((ConfigOptionBool, zaa_enabled))
+    ((ConfigOptionBool, zaa_dont_alternate_fill_direction))
+    ((ConfigOptionFloat, zaa_min_z))
+
     // Orca: internal use only
     ((ConfigOptionBool,  calib_flowrate_topinfill_special_order)) // ORCA: special flag for flow rate calibration
 )
@@ -1101,6 +1106,7 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionFloat, ironing_speed))
     ((ConfigOptionFloat, ironing_angle))
     ((ConfigOptionBool, ironing_angle_fixed))
+    ((ConfigOptionFloat, ironing_expansion))
     // Filament Ironing
     ((ConfigOptionPercentsNullable, filament_ironing_flow))
     ((ConfigOptionFloatsNullable, filament_ironing_spacing))
@@ -1189,6 +1195,10 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionFloatOrPercent,       scarf_joint_speed))
     ((ConfigOptionFloat,                scarf_joint_flow_ratio))
     ((ConfigOptionPercent,              scarf_overhang_threshold))
+
+    // Z Anti-Aliasing (aka Z Contouring)
+    ((ConfigOptionBool, zaa_region_disable))
+    ((ConfigOptionFloat, zaa_minimize_perimeter_height))
 )
 
 PRINT_CONFIG_CLASS_DEFINE(