fix(constraints): point symmetry converges from any start

Symmetric held the connecting direction perpendicular to the axis with
the kernel's angle-based Perpendicular, which locks its turn side at
construction: a point starting far from its mirror image had to swing
the pair through the axis direction, the residual wrapped, and the
solver stalled with the point unmoved. The perpendicularity is now the
projection of the pair onto the axis direction being zero, a polynomial
residual the solver follows from anywhere. Circle centers and ellipse
centers use the same equation. tests/parametric_constraints_solve.rs
symmetric_constraint_mirrors_one_circles_center_across_the_axis_line
passes again.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/scene/parametric_solve.rs

@@ -1466,13 +1466,23 @@ fn build_constraint(
             let Some(mirror) = whole_line(sys, cache, *m) else {
                 return Vec::new();
             };
+            // The connecting direction is held perpendicular to the axis as a
+            // projection onto the axis direction being zero: a polynomial
+            // residual the solver can follow from any start. The angle form
+            // (`Perpendicular`) locks its turn side at construction and stalls
+            // when the pair has to swing through the axis direction — a point
+            // starting far from its mirror image never arrived.
+            let across_axis = |sys: &mut System, pa: GPoint, pb: GPoint| -> Rc<dyn Constraint> {
+                let zero = sys.add_param(0.0, true);
+                Rc::new(ProjectedDistanceAlongLine::new(pa, pb, zero, mirror, false))
+            };
             if let (Some(pa), Some(pb)) =
                 (point_ref(sys, cache, *a), point_ref(sys, cache, *b))
             {
                 let pair = GLine { p1: pa, p2: pb };
                 return vec![
                     Rc::new(MidpointOnLine::new(pair, mirror)),
-                    Rc::new(PerpendicularConstraint::new(sys.store(), pair, mirror)),
+                    across_axis(sys, pa, pb),
                 ];
             }
             let (Some(first), Some(second)) = (
@@ -1500,11 +1510,7 @@ fn build_constraint(
                     };
                     vec![
                         Rc::new(MidpointOnLine::new(centers, mirror)),
-                        Rc::new(PerpendicularConstraint::new(
-                            sys.store(),
-                            centers,
-                            mirror,
-                        )),
+                        across_axis(sys, first.center, second.center),
                         Rc::new(Equal::new(second.rad, first.rad, 1.0)),
                     ]
                 }
@@ -1523,11 +1529,7 @@ fn build_constraint(
                     };
                     vec![
                         Rc::new(MidpointOnLine::new(centers, mirror)),
-                        Rc::new(PerpendicularConstraint::new(
-                            sys.store(),
-                            centers,
-                            mirror,
-                        )),
+                        across_axis(sys, first.center, second.center),
                         Rc::new(SymmetricLineDirections::new(
                             sys.store(),
                             first_axis,