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/OptionsGroup.cpp

@@ -621,7 +621,7 @@ void OptionsGroup::on_change_OG(const t_config_option_key& opt_id, const boost::
 Option ConfigOptionsGroup::get_option(const std::string& opt_key, int opt_index /*= -1*/)
 {
 	if (!m_config->has(opt_key)) {
-		std::cerr << "No " << opt_key << " in ConfigOptionsGroup config.\n";
+		// Option not in config — may be newly added (e.g. ZAA options)
 	}
 
 	std::string opt_id = opt_index == -1 ? opt_key : opt_key + "#" + std::to_string(opt_index);