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.
Add half-space tracking for all compound constraints with branch
ambiguity: Planar, Revolute, Concentric, Cylindrical, Slider, Screw,
Universal, PointInPlane, and LineInPlane. Previously only
DistancePointPoint, Parallel, Angle, and Perpendicular were tracked,
so the Newton-Raphson solver could converge to the wrong branch for
compound constraints — causing parts to drift through plane
constraints while honoring revolute joints.
Add quaternion continuity enforcement in drag_step(): after solving,
each non-dragged body's quaternion is checked against its pre-step
value and negated if in the opposite hemisphere (standard SLERP
short-arc correction). This prevents the C++ validateNewPlacements()
from rejecting valid solutions as 'flipped orientation' due to the
quaternion double-cover ambiguity (q and -q encode the same rotation
but measure as ~340° apart).
