feat: SaltyRover 4-wheel diff-drive control loop (#74)
Adds saltyrover_drive ROS2 package — 4-wheel differential drive controller
for the stable SaltyRover chassis (no balance PID required).
rover_drive_node:
- Subscribes /cmd_vel (Twist), converts to left/right wheel speeds
- Tank-style kinematics: v_left = linear.x - angular.z × sep/2
- FL/RL share left speed, FR/RR share right speed (paired axles)
- Publishes /rover/wheel_speeds (JSON: {"cmd":"drive4","fl":N,"fr":N,"rl":N,"rr":N})
- Trapezoidal ramp: accel 2.0 m/s², decel 4.0 m/s² (stable chassis allows faster ramps)
- cmd_vel watchdog (0.5s timeout → zero speeds)
- max_speed 3.0 m/s (±1000 STM32 ticks)
rover_odom_node:
- Dead-reckoning odometry from /cmd_vel (encoder-ready for future HW)
- Publishes /odom (nav_msgs/Odometry) + TF odom→base_link
- Forward Euler pose integration at 50 Hz
52/52 unit tests: kinematics, int scaling, JSON builder, full pipeline,
trapezoidal ramp, odometry integration, quaternion conversion.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
sl-controls
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