feat: various multibody/mjcf fixes

[sebcrozet]

Added

• Multibody joints now support a per-DoF armature (reflected rotor inertia, matching MuJoCo's
  concept), exposed via Multibody::armature() / armature_mut(). The related
  Multibody::dof_inverse_inertia() returns the articulated DoF inverse inertia including armature.
• Multibody joints now support passive per-DoF springs (MuJoCo-style spring/damper) via
  MultibodyJoint::set_spring() / spring(), integrated implicitly.
• Multibody joints now support holonomic DoF coupling: constrain one generalized coordinate to
  another via MultibodyDofCoupling and Multibody::add_dof_coupling() / couplings().
• MultibodyLink::assembly_id() exposes the offset of a link's DoFs in the multibody's generalized
  coordinate/velocity vectors.
• rapier3d-mjcf: support for tendons, equality constraints, materials (including visual-mesh
  materials), actuator max-force, joint DoF coupling, armature, per-DoF spring/damper, and control.
• Testbed: the ability to specify a material.

Fixed

• Corrected the local frame of impulse joints attached to multibodies.
• Added a missing coupling term for loop-closing impulse-joint constraints (bodies with a non-zero
  center-of-mass offset).
• Fixed a crash occurring with the parallel feature enabled.