arctan2(dy, dx) gives east-based math bearing; Tank Royale uses north=0°, CW+.
Swapping to arctan2(dx, dy) gives the correct game-space bearing.
Symptom: radar commanded 45°/tick away from a target directly ahead.
Adds test_radar_lock.nim as regression test (20-tick lock, ±15° tolerance).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>