feat: Add Z Anti-Aliasing (ZAA) contouring support (updated) (#12736)

This PR supersedes #12225, which originally proposed this feature but
appears inactive.

The feature originated from work I developed earlier in
[BambuStudio-ZAA](https://github.com/adob/BambuStudio-ZAA), a private
fork of Bambu Studio

Compared to #12225, I updated the implementation for current upstream
and fixed the following issues:
 - fixed broken tests
 - removed references to nonplanar directory

Reviewers may want to compare against #12225 for earlier
discussion/context.

## Summary

Port of **Z Anti-Aliasing (ZAA)** from
[BambuStudio-ZAA](https://github.com/adob/BambuStudio-ZAA) to
OrcaSlicer.

ZAA eliminates visible stair-stepping on curved and sloped top surfaces
by raycasting each extrusion point against the original 3D mesh and
micro-adjusting its Z height to follow the actual surface geometry. The
result is visibly smoother domes, chamfers, and shallow slopes — without
post-processing.

## How It Works

1. The slicer runs normally, then a **posContouring** step processes
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 with per-point Z coordinates
4. The G-code writer emits the adjusted Z values, so the printer follows
the true surface

## Configuration

Five new 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; controls
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 |
| `zaa_region_disable` | bool | off | Disable ZAA for a specific print
region/material |

## Key Changes

- **Core algorithm**: New `src/libslic3r/ContourZ.cpp` (~330 lines) —
raycasting engine
- **3D geometry**: `Point3`, `Line3`, `Polyline3`, `MultiPoint3` extend
existing 2D types
- **Arc fitting**: Templated to work with both 2D and 3D geometry
- **Pipeline**: `ExtrusionPath::polyline` changed from `Polyline` to
`Polyline3`; new `posContouring` step in `PrintObject.cpp`
- **G-code**: `GCode.cpp` writes per-point Z when `path.z_contoured` is
set
- **UI**: ZAA settings exposed in Print Settings > Quality panel
- **Documentation**: `docs/ZAA.md` with usage and implementation details

57 files changed, ~1800 insertions, ~200 deletions.

## Test Plan

- [ ] Load a model with curved top surfaces (sphere, dome, chamfered
box)
- [ ] Enable **Z contouring** in Print Settings > Quality
- [ ] Slice and verify G-code has varying Z values within contoured
layers
- [ ] Build on macOS (verified), test on Linux and Windows

src/libslic3r/PrintConfig.cpp

@@ -4111,6 +4111,51 @@ 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     = comExpert;
+    def->set_default_value(new ConfigOptionFloat(0));
+
+    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     = comExpert;
+    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 height of top surface perimeters to match height of model edge. "
+                       "Effects perimeters whose slope is less than this angle in degrees. Reasonable value is 35. Set 0 to disable.");
+    def->sidetext = L("°");
+    def->min      = 0;
+    def->max      = 90;
+    def->mode     = comExpert;
+    def->set_default_value(new ConfigOptionFloat(0));
+
+    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     = comExpert;
+    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     = comExpert;
+    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.");