fix: remove planar half-space correction for on-plane distance=0 constraints

When a PlanarConstraint has distance=0 (point already on plane), the
half-space tracker was using dot(z_i, z_j) as its indicator and providing
a correction function that reflects the body through the plane.

When combined with a Cylindrical joint, legitimate rotation about the
cylinder axis causes the body's planar face normal to rotate. After ~90
degrees, the dot product crosses zero, triggering the correction which
teleports the body to the other side of the plane. The C++ validator
then rejects every subsequent drag step as 'flipped orientation'.

Fix: return a tracking-only HalfSpace (no correction_fn) when the point
is already on the plane. This matches the pattern used by Cylindrical,
Revolute, and Concentric half-space trackers. The cross-product residuals
in PlanarConstraint already enforce normal alignment via the solver
itself, so the correction is redundant and harmful during drag.

kindred_solver/preference.py

@@ -283,11 +283,18 @@ def _planar_half_space(
     # Use the normal dot product as the primary indicator when the point
     # is already on the plane (distance ≈ 0).
     if abs(d_val) < 1e-10:
-        # Point is on the plane — track normal direction instead
+        # Point is on the plane — track normal direction only.
+        # No correction: when combined with rotational joints (Cylindrical,
+        # Revolute), the body's marker normal rotates legitimately and
+        # reflecting through the plane would fight the rotation.
         def indicator(e: dict[str, float]) -> float:
             return dot_expr.eval(e)
 
-        ref_sign = normal_ref_sign
+        return HalfSpace(
+            constraint_index=constraint_idx,
+            reference_sign=normal_ref_sign,
+            indicator_fn=indicator,
+        )
     else:
         # Point is off-plane — track which side
         def indicator(e: dict[str, float]) -> float: