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/ExtrusionSimulator.cpp

@@ -962,7 +962,7 @@ void ExtrusionSimulator::extrude_to_accumulator(const ExtrusionPath &path, const
 	w = scale_(path.mm3_per_mm / path.height) * scalex;
 	// printf("scalex: %f, scaley: %f\n", scalex, scaley);
 	// printf("bbox: %d,%d %d,%d\n", bbox.min.x(), bbox.min.y, bbox.max.x(), bbox.max.y);
-	for (Points::const_iterator it = path.polyline.points.begin(); it != path.polyline.points.end(); ++ it) {
+	for (Points3::const_iterator it = path.polyline.points.begin(); it != path.polyline.points.end(); ++ it) {
 		// printf("point %d,%d\n", it->x+shift.x(), it->y+shift.y);
 		ExtrusionPoint ept;
 		ept.center = V2f(float((*it)(0)+shift.x()-bbox.min.x()) * scalex, float((*it)(1)+shift.y()-bbox.min.y()) * scaley);