Adds console_test_planar_drag.py — a live FreeCAD console test that
reproduces the quaternion branch-jump failure from #338.
Test 2 (realistic geometry) reliably triggers the bug: 10/40 drag
steps rejected by the C++ validateNewPlacements() simulator when
the solver converges to an equivalent but distinct quaternion branch
around 240-330 deg axial rotation.
Key findings from the test:
- The failure is NOT simple hemisphere negation (q vs -q)
- The solver finds geometrically valid but quaternion-distinct
solutions when Cylindrical + Planar constraints have multiple
satisfying orientations
- _enforce_quat_continuity only catches sign flips, not these
deeper branch jumps
- The C++ validator uses acos(w) not acos(|w|), so opposite-
hemisphere quaternions show as ~360 deg rotation
9e07ef8679