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.

docs/ZAA.md

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+# Z Anti-Aliasing (ZAA) — Z Contouring
+
+ZAA eliminates stair-stepping on curved and sloped top surfaces by adjusting the Z height of each extrusion point to follow the actual 3D model surface.
+
+Instead of printing flat horizontal layers, ZAA raycasts each point of the toolpath against the original mesh and micro-adjusts its Z coordinate to match the true surface geometry. The result is visibly smoother surfaces on domes, chamfers, and shallow slopes — without post-processing.
+
+This is a port of the ZAA implementation from [BambuStudio-ZAA](https://github.com/adob/BambuStudio-ZAA) by adob.
+
+## Configuration
+
+ZAA adds five settings under **Print Settings > Quality**:
+
+| Setting | Type | Default | Description |
+|---------|------|---------|-------------|
+| `zaa_enabled` | bool | off | Master enable/disable switch |
+| `zaa_min_z` | float | 0.06 mm | Minimum Z layer height; also controls the slicing plane offset |
+| `zaa_minimize_perimeter_height` | float | 35° | Reduce perimeter heights on slopes below this angle (0 = disabled) |
+| `zaa_dont_alternate_fill_direction` | bool | off | Keep fill direction consistent instead of alternating per layer |
+| `zaa_region_disable` | bool | off | Disable ZAA for a specific print region/material |
+
+## How It Works
+
+1. The slicer slices normally, then runs a **posContouring** step on each layer.
+2. `ContourZ.cpp` raycasts every extrusion point vertically against the source mesh.
+3. Each point's Z is adjusted to the mesh intersection, converting flat `Polyline` paths into `Polyline3` paths that carry per-point Z coordinates.
+4. The G-code writer emits the adjusted Z values, so the printer follows the true surface.
+
+## Key Implementation Details
+
+- **Core algorithm**: `src/libslic3r/ContourZ.cpp` (~330 lines)
+- **3D geometry**: `Point3`, `Line3`, `Polyline3`, `MultiPoint3` extend the existing 2D types
+- **Pipeline step**: `posContouring` in `PrintObject.cpp`, runs after perimeter/infill generation
+- **G-code output**: `GCode.cpp` writes per-point Z when `path.z_contoured` is set
+- **Arc fitting**: Templated to work with both 2D and 3D geometry
+- **ExtrusionPath change**: `polyline` field changed from `Polyline` to `Polyline3`
+
+## Testing
+
+1. Load a model with curved top surfaces (spheres, domes, chamfered edges)
+2. Enable **Z contouring** in Print Settings > Quality
+3. Slice and inspect the G-code — Z values should vary within each layer on contoured surfaces