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.

version.inc

@@ -8,6 +8,7 @@ if(NOT DEFINED BBL_INTERNAL_TESTING)
 set(BBL_INTERNAL_TESTING "0")
 endif()
 set(SoftFever_VERSION "2.3.2-dev")
+set(ZAA_VERSION "1.0.3")
 string(REGEX MATCH "^([0-9]+)\\.([0-9]+)\\.([0-9]+)"
        SoftFever_VERSION_MATCH ${SoftFever_VERSION})
 set(ORCA_VERSION_MAJOR ${CMAKE_MATCH_1})