Adds face_track_servo_node to saltybot_social:
- Subscribes /social/faces/detected (FaceDetectionArray)
- Picks closest face by largest bbox area (proximity proxy)
- Computes pan/tilt error from bbox centre vs image centre using
configurable FOV (fov_h_deg=60°, fov_v_deg=45°)
- Independent PID controllers for pan and tilt (velocity/incremental
output with anti-windup); servo position integrates velocity*dt
- Clamps commands to ±pan_limit_deg / ±tilt_limit_deg
- Returns to centre at return_rate_deg_s when face lost >lost_timeout_s
- Dead zone suppresses jitter for small errors
- Publishes Float32 on /saltybot/head_pan and /saltybot/head_tilt
- 81/81 tests passing
Closes#279
