Fix cartpole dynamics equations

[devshahofficial]

Summary

• Correct the cartpole dynamics equations for pole mass length and theta acceleration.
• Add a C-level regression that compiles cartpole.h and validates c_step against the reference equations.

Root cause

polemass_length used total_mass + pole_mass instead of pole_mass * pole_length, and the theta acceleration denominator canceled total_mass instead of scaling by pole_mass / total_mass.

Validation

• python -m pytest tests/test_cartpole_dynamics.py tests/test_import_performance.py -q

Ports the active fix for the 4.0 branch.

Fixes #447