Unify colinear simplify tolerance in Arachne (#15314)

Introduced a shared `colinear_vertex_tolerance()` helper in `ExtrusionLine.hpp` and updated both simplify paths (`ExtrusionLine.cpp` and `WallToolPaths.cpp`) to use it instead of duplicated hardcoded `0.005` scaled thresholds. This keeps the near-colinear early-out tied to `SCALED_EPSILON` (rounding-noise scale) and avoids unintended curve decimation from larger tolerances, while documenting the geometric impact in code.

src/libslic3r/Arachne/WallToolPaths.cpp

@@ -154,8 +154,8 @@ void simplify(Polygon &thiss, const int64_t smallest_line_segment_squared, const
         //h^2 = L^2 / b^2     [factor the divisor]
         const int64_t height_2 = double(area_removed_so_far) * double(area_removed_so_far) / double(base_length_2);
         // Orca: The value of `height_2` is squared, so we need to compare it with the squared value
-        if ((height_2 <= Slic3r::sqr(scaled<coord_t>(0.005)) //Almost exactly colinear (barring rounding errors).
+        if ((height_2 <= Slic3r::sqr(colinear_vertex_tolerance()) //Almost exactly colinear (barring rounding errors).
-             && Line::distance_to_infinite(current, previous, next) <= scaled<double>(0.005))) // make sure that height_2 is not small because of cancellation of positive and negative areas
+             && Line::distance_to_infinite(current, previous, next) <= double(colinear_vertex_tolerance()))) // make sure that height_2 is not small because of cancellation of positive and negative areas
             continue;
 
         if (length2 < smallest_line_segment_squared

src/libslic3r/Arachne/utils/ExtrusionLine.cpp

@@ -133,8 +133,8 @@ void ExtrusionLine::simplify(const int64_t smallest_line_segment_squared, const
         const auto    height_2 = int64_t(double(area_removed_so_far) * double(area_removed_so_far) / double(base_length_2));
         const int64_t extrusion_area_error = calculateExtrusionAreaDeviationError(previous, current, next);
         // Orca: The value of `height_2` is squared, so we need to compare it with the squared value
-        if ((height_2 <= Slic3r::sqr(scaled<coord_t>(0.005)) // Almost exactly colinear (barring rounding errors).
+        if ((height_2 <= Slic3r::sqr(colinear_vertex_tolerance()) // Almost exactly colinear (barring rounding errors).
-             && Line::distance_to_infinite(current.p, previous.p, next.p) <= scaled<double>(0.005)) // Make sure that height_2 is not small because of cancellation of positive and negative areas
+             && Line::distance_to_infinite(current.p, previous.p, next.p) <= double(colinear_vertex_tolerance())) // Make sure that height_2 is not small because of cancellation of positive and negative areas
             // We shouldn't remove middle junctions of colinear segments if the area changed for the C-P segment is exceeding the maximum allowed
              && extrusion_area_error <= maximum_extrusion_area_deviation)
         {

src/libslic3r/Arachne/utils/ExtrusionLine.hpp

@@ -32,6 +32,14 @@ class Flow;
 namespace Slic3r::Arachne
 {
 
+// ORCA: Tolerance of the "almost exactly colinear" early-out shared by the two simplify() passes
+// (this file and WallToolPaths.cpp). That test drops a vertex regardless of the user's Maximum wall
+// resolution/deviation, so it has to stay at the scale of coordinate rounding noise. A larger value
+// silently decimates finely tessellated curves: on a circle, one vertex may be removed whenever the
+// sagitta of the resulting chord falls below the tolerance, which halves the point count and turns
+// smooth arcs into corners the firmware has to decelerate through.
+inline coord_t colinear_vertex_tolerance() { return coord_t(SCALED_EPSILON); }
+
 /*!
  * Represents a polyline (not just a line) that is to be extruded with variable
  * line width.