fix(solver): redirect distance=0 constraint to CoincidentConstraint

DistancePointPointConstraint uses a squared residual (||p_i-p_j||^2 - d^2)
which has a degenerate Jacobian when d=0 and the constraint is satisfied
(all partial derivatives vanish). This made the constraint invisible to
the Newton solver during drag, allowing constrained points to drift apart.

When distance=0, use CoincidentConstraint instead (3 linear residuals:
dx, dy, dz) which always has a well-conditioned Jacobian.

kindred_solver/solver.py

@@ -538,6 +538,16 @@ def _build_constraint(
 
     if kind == kcsolve.BaseJointKind.DistancePointPoint:
         distance = c_params[0] if c_params else 0.0
+        if distance == 0.0:
+            # Distance=0 is point coincidence.  Use CoincidentConstraint
+            # (3 linear residuals) instead of the squared form which has
+            # a degenerate Jacobian when the constraint is satisfied.
+            return CoincidentConstraint(
+                body_i,
+                marker_i_pos,
+                body_j,
+                marker_j_pos,
+            )
         return DistancePointPointConstraint(
             body_i,
             marker_i_pos,