Add LiDAR-based wall-following controller

[Aeh961]

Summary

Closes #6.

Adds a classical PD-style controller that maintains a fixed standoff distance from a configurable wall side using the existing eight-beam LiDAR ring.

No MJCF, sensor, vehicle-physics, or track-geometry changes are included.

How it works

The controller uses two primary signals:

• distance_error: difference between the measured side distance and the configured target distance
• wall_skew: difference between the rear-side and front-side beam readings, used to estimate the car's angle relative to the wall

The signals use the same meaning for either wall side. A single directional sign is applied for follow_side="left" or follow_side="right", making the behaviors mirror images.

The controller also:

• reduces throttle when steering sharply
• reduces throttle when an obstacle is detected ahead
• handles invalid, missing, and non-finite LiDAR readings safely
• clamps steering output
• supports steering slew-rate limiting
• exposes reset() for controller state

Changes

• Added examples/wall_following.py

  • WallFollowingConfig
  • ControlCommand
  • WallFollowingSignals
  • compute_signals()
  • WallFollowingController

• Updated examples/sensor_logger.py

  • moved reusable LiDAR beam-grouping constants into the module that owns the LiDAR beam order and angles

• Updated examples/track_centering.py

  • imports the shared beam constants from sensor_logger.py
  • no controller behavior changed

• Added validation/test_wall_following.py

  • 13 focused tests

Test coverage

The tests cover:

• left-side and right-side mirror behavior
• too-close and too-far corrections
• neutral behavior at the target distance
• wall-angle correction
• obstacle-based throttle reduction
• sharp-steering throttle reduction
• missing and non-finite reading fallback
• no-hit LiDAR handling
• steering clamping
• slew-rate limiting
• controller reset
• one physics integration test using straight_corridor.xml

Validation

• pytest validation/test_wall_following.py: 13/13 passed
• pytest validation/: 47/47 passed
• Ruff check and format check pass on the new files

The full-repository Ruff command continues to report the same pre-existing errors caused by the repository's Ruff version drift. This branch does not introduce additional Ruff violations.

Limitations

• The default gains are reasonable starting values and may require additional tuning for other tracks or driving speeds.
• The followed wall side is fixed through configuration and does not automatically switch if the wall disappears.
• End-to-end physics validation currently covers the straight corridor; curved-track behavior is covered through the controller's pure signal and steering tests.