Implement Quadrotor plant (12D state, 4D control)

[adilfaisal01]

src/shinro/plants/quadrotor.py is a registered placeholder — every method raises NotImplementedError. Only TODO in the codebase.

Spec (from class docstring):

• State: 12D (pose + twist)
• Control: 4D (thrust + body torques) — higher-level abstraction TBD
• Follows the HolonomicMobileRobot pattern
• Needs: standalone analytical dynamics + MuJoCo engine mode

Reference plants: double_pendulum.py (analytical dynamics), holonomicmobilerobot.py, physics_engine/mujoco.py.

Priority: P3.