Replace ImplicitActuator (which doesn't work for prismatic joints) with
IdealPDActuator. This bypasses PhysX drive and explicitly computes PD forces
using set_dof_actuation_forces().
Changes:
- stiffness=100k, damping=5k to resist arm reaction forces (~150N) with <2mm deflection
- Read trunk_cmd from ArticulationCfg initial position (single source of truth)
- Integrate trunk diagnostics into robot state output
- Remove hardcoded trunk_target parameter