[plancherb1]

• The legacy floating-base convention plumbing is gone end to end: the private normalize/denormalize helpers, _permute_matrix_prefix, and the normalize_input / public_output keywords on the dynamics entry points; xfail_if_mimic and the convention test deleted.
• robot_assets/signed_axis_arm.urdf (+Z, -Z, +X, -Y revolute axes, base inertia) joins the smoke tier with a coverage test (GATO issue 22).
• The Pinocchio backend's pose gradient is analytic (frame Jacobian plus the rpy rate map) and its pose Hessian one central difference of it; the installed Pinocchio has no kinematic-Hessian binding, so the old path differenced a differenced gradient at a 1e-5 noise floor. New tests/test_pinocchio_pose_derivatives.py.
• Smooth hinge activation (kind 4), state_cost_tangent for the floating-base menu, tangent-sized frame terms, the frame velocity derivative along the tangent on floating bases; a duplicated 166-line copy of the plant mixin's frame and cost methods removed.
• Suite: 961 pass, 1 pre-existing skip.

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