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/PrintObject.cpp

@@ -8,6 +8,7 @@
 #include "Layer.hpp"
 #include "MutablePolygon.hpp"
 #include "PrintConfig.hpp"
+#include "SLA/IndexedMesh.hpp"
 #include "Support/SupportMaterial.hpp"
 #include "Support/SupportSpotsGenerator.hpp"
 #include "Support/TreeSupport.hpp"
@@ -23,6 +24,10 @@
 #include "AABBTreeLines.hpp"
 
 #include <float.h>
+#include <iterator>
+#include <mutex>
+#include <ostream>
+#include <string>
 #include <oneapi/tbb/blocked_range.h>
 #include <oneapi/tbb/concurrent_vector.h>
 #include <oneapi/tbb/parallel_for.h>
@@ -709,6 +714,46 @@ void PrintObject::ironing()
     }
 }
 
+void PrintObject::contour_z()
+{
+    if (!this->set_started(posContouring)) {
+        return;
+    }
+
+    m_print->set_status(40, L("Z contouring"));
+    BOOST_LOG_TRIVIAL(debug) << "Contouring in parallel - start";
+
+    TriangleMesh mesh = this->m_model_object->raw_mesh();
+    if (m_model_object->instances.size() != 1) {
+        throw RuntimeError("ContourZ: unexpected number of instances");
+    }
+
+    m_model_object->instances.front()->transform_mesh(&mesh, true);
+    sla::IndexedMesh imesh(mesh);
+
+    std::mutex mtx;
+    size_t completed = 0;
+    tbb::parallel_for(
+        // Contouring starting with layer second layer to avoid build plate collision
+        tbb::blocked_range<size_t>(1, m_layers.size()),
+        [&, this](const tbb::blocked_range<size_t>& range) {
+            for (size_t layer_idx = range.begin(); layer_idx < range.end(); layer_idx++) {
+                m_print->throw_if_canceled();
+                m_layers[layer_idx]->make_contour_z(imesh);
+
+                std::scoped_lock lock(mtx);
+                completed++;
+                std::string msg = (boost::format("Z contoured layer %d/%d (%d%%)") % (completed) % m_layers.size() % int(double(completed) / m_layers.size() * 100)).str();
+                m_print->set_status(40, msg);
+            }
+        }
+    );
+    m_print->throw_if_canceled();
+    BOOST_LOG_TRIVIAL(debug) << "Contouring in parallel - end";
+
+    this->set_done(posContouring);
+}
+
 // BBS
 void PrintObject::clear_overhangs_for_lift()
 {
@@ -1351,15 +1396,15 @@ bool PrintObject::invalidate_step(PrintObjectStep step)
 
     // propagate to dependent steps
     if (step == posPerimeters) {
-		invalidated |= this->invalidate_steps({ posPrepareInfill, posInfill, posIroning, posSimplifyPath, posSimplifyInfill });
+		invalidated |= this->invalidate_steps({ posPrepareInfill, posInfill, posIroning, posContouring, posSimplifyPath, posSimplifyInfill });
         invalidated |= m_print->invalidate_steps({ psSkirtBrim });
     } else if (step == posPrepareInfill) {
-        invalidated |= this->invalidate_steps({ posInfill, posIroning, posSimplifyPath, posSimplifyInfill });
+        invalidated |= this->invalidate_steps({ posInfill, posIroning, posContouring, posSimplifyPath, posSimplifyInfill });
     } else if (step == posInfill) {
-        invalidated |= this->invalidate_steps({ posIroning, posSimplifyInfill });
+        invalidated |= this->invalidate_steps({ posIroning, posContouring, posSimplifyInfill });
         invalidated |= m_print->invalidate_steps({ psSkirtBrim });
     } else if (step == posSlice) {
-		invalidated |= this->invalidate_steps({ posPerimeters, posPrepareInfill, posInfill, posIroning, posSupportMaterial, posSimplifyPath, posSimplifyInfill });
+		invalidated |= this->invalidate_steps({ posPerimeters, posPrepareInfill, posInfill, posIroning, posContouring, posSupportMaterial, posSimplifyPath, posSimplifyInfill });
         invalidated |= m_print->invalidate_steps({ psSkirtBrim });
         m_slicing_params.valid = false;
     } else if (step == posSupportMaterial) {