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.cpp

@@ -4005,6 +4005,57 @@ void PrintConfigDef::init_fff_params()
     def->mode     = comAdvanced;
     def->set_default_value(new ConfigOptionBool(false));
 
+    def           = this->add("ironing_expansion", coFloat);
+    def->label    = L("Ironing expansion");
+    def->category = L("Quality");
+    def->tooltip  = L("Expand or contract the ironing area.");
+    def->sidetext = L("mm");
+    def->min      = -100;
+    def->max      = 100;
+    def->mode     = comAdvanced;
+    def->set_default_value(new ConfigOptionFloat(0));
+
+    def = this->add("zaa_region_disable", coBool);
+    def->label    = L("Disable Z contouring for region");
+    def->category = L("Quality");
+    def->tooltip  = L("Disable Z contouring for this specific region");
+    def->mode     = comAdvanced;
+    def->set_default_value(new ConfigOptionBool(false));
+
+    def = this->add("zaa_enabled", coBool);
+    def->label    = L("Z contouring enabled");
+    def->category = L("Quality");
+    def->tooltip  = L("Enable Z-layer contouring (aka Z-layer anti-aliasing)");
+    def->mode     = comAdvanced;
+    def->set_default_value(new ConfigOptionBool(false));
+
+    def = this->add("zaa_minimize_perimeter_height", coFloat);
+    def->label    = L("Minimize wall height angle");
+    def->category = L("Quality");
+    def->tooltip  = L("Reduce top surface perimeter heights to match height of edge for perimeters less than this angle. Set 0 to disable.");
+    def->sidetext = L("°");
+    def->min      = 0;
+    def->max      = 90;
+    def->mode     = comAdvanced;
+    def->set_default_value(new ConfigOptionFloat(35));
+
+    def = this->add("zaa_dont_alternate_fill_direction", coBool);
+    def->label    = L("Don't alternate fill direction");
+    def->category = L("Quality");
+    def->tooltip  = L("Disable alternating fill direction when using Z contouring");
+    def->mode     = comAdvanced;
+    def->set_default_value(new ConfigOptionBool(false));
+
+    def = this->add("zaa_min_z", coFloat);
+    def->label    = L("Minimum z height");
+    def->category = L("Quality");
+    def->tooltip  = L("Minimum z layer height. Also controls slicing plane");
+    def->sidetext = L("mm");
+    def->min      = 0;
+    def->max      = 100;
+    def->mode     = comAdvanced;
+    def->set_default_value(new ConfigOptionFloat(0.05));
+
     def = this->add("layer_change_gcode", coString);
     def->label = L("Layer change G-code");
     def->tooltip = L("This G-code is inserted at every layer change after the Z lift.");