Add IPC contact force recording and improve transform synchronization

- Add coupling_strategy option (two_way_soft_constraint, contact_proxy)
- Move _genesis_stored_states from rigid_solver to ipc_coupler
- Implement _record_ipc_contact_forces() to extract contact forces from IPC
- Implement _apply_ipc_contact_forces() to apply forces to Genesis links
- Refactor _set_genesis_transforms_from_ipc() to support multi-link IK
  - Use inverse_kinematics_multilink for complex entities
  - Direct base link setting for simple single-body entities
- Add strategy-based dispatching in substep_pre/post_coupling

Author: Roushelfy

examples/IPC_Solver/genesis_ipc_motion_test.py

@@ -296,10 +296,6 @@ def compute_total_linear_momentum():
     ax.grid(True, alpha=0.3)
 
     plt.tight_layout()
-    plt.savefig("momentum_conservation_test.png", dpi=150)
-    print(f"\n{'='*70}")
-    print("Plot saved to: momentum_conservation_test.png")
-    print(f"{'='*70}")
     plt.show()