[codex] Add dynamics C++ backend and benchmarks

[yyyyyt123]

Summary

• Add a C++/Eigen fixed-base dynamics backend for RNEA, CRBA, gravity, nonlinear effects, and forward dynamics via mass-matrix solve.
• Add reproducible benchmark CLI commands for kinematics and dynamics, including JSON/Markdown outputs and CI smoke coverage.
• Record a v2.6.0 dynamics CPU benchmark baseline against RoboCore NumPy and Pinocchio.
• Update README/docs/examples/tests around backend selection, dynamics limits, and benchmark policy.

Notes

• Forward dynamics is documented as M(q) ddq = tau - nle(q, v), not true O(n) ABA yet.
• Pinocchio is used as an optional external comparison and validation dependency.

Validation

• python -m compileall -q robocore examples test
• pytest test/unit/test_dynamics.py test/integration/test_benchmark_cli.py test/integration/test_dynamics_pinocchio.py -q -> 13 passed
• pytest test/unit test/integration -q -> 242 passed, 27 skipped
• python -m robocore.benchmark dynamics --backends cpp,numpy,pinocchio --samples 200 --repeats 5 --inner-single 1000 --inner-batch 200 --output-dir benchmark-results/v2.6.0-dynamics-cpu
• python -m build
• python -m twine check dist/*