improvement: rewrite calibration as manual range tracking

Replace the automated stall-detection calibration with a manual
approach: disable torque on all servos, let the user move each joint
through its full range while tracking min/max positions in real time,
then calculate centre offsets.

The manual approach is more reliable — stall detection was fragile
and risked crashing the arm into obstacles. The new implementation
handles the 0/4095 position wraparound correctly using unwrapped
position tracking, and resets angle limits to 0/4095 to avoid
inverted-limit firmware issues.

Author: jimsynz