point-arc coincident jankiness

[adamchalmers]

The point-arc coincident constrained added in #195 has some janky behaviour:

Screenshare.-.2025-12-18.5_51_03.PM.mp4

@settings(experimentalFeatures = allow)

sketch(on = XZ) {
  point1 = sketch2::point(at = [0mm, 0mm])
  arc1 = sketch2::arc(start = [var -0mm, var 5.05mm], end = [var -5.05mm, var -0mm], center = [var -0mm, var 0mm])
  sketch2::coincident([arc1.center, point1])
  line1 = sketch2::line(start = [var -5.05mm, var -0mm], end = [var -0mm, var -0mm])
  line2 = sketch2::line(start = [var -0mm, var 0mm], end = [var -0mm, var 5.05mm])
  sketch2::coincident([line2.end, arc1.start])
  sketch2::coincident([arc1.end, line1.start])
  sketch2::coincident([line1.end, point1])
  sketch2::coincident([point1, line2.start])
  sketch2::vertical(line2)
  sketch2::horizontal(line1)
sketch2::distance([line1.start, line1.end]) == 5.05mm
  line3 = sketch2::line(start = [var 14.64mm, var 1.3mm], end = [var -4.968mm, var 0.907mm])
  sketch2::coincident([line3.end, arc1])
}

Figure out why.