fix: symmetric calibration for wrist_roll motors in SO-101

[cwzhong627-source]

fix(robots): ensure symmetric wrist_roll calibration for SO-101

Type / Scope

• Type: Bug
• Scope: lerobot/robots/so_follower, lerobot/teleoperators/so_leader

Summary / Motivation

Current calibration for wrist_roll (full-turn motors) in SO-101 robots relies on a hardcoded range or an asymmetric logic, which causes the software "zero" point to deviate from the physical orientation set by the user during the calibration process.

This PR implements a symmetric range strategy. By calculating a safe_delta centered at the current physical position, we ensure that the software center ((min + max) / 2) perfectly aligns with the user-defined physical zero. This fixes the "offset wrist" issue and ensures consistent range of motion in both directions.

Related issues

Fixes / Closes: #3193

What changed

• Modified so_follower.py and so_leader.py to read current_pos of the full_turn_motor before recording other joints.
• mplemented safe_delta = min(current_pos, 4095 - current_pos, 2048) to calculate strictly symmetric range_min and range_max.
• Breaking changes: None. This improves calibration accuracy for existing SO-101 users.

How was this tested (or how to run locally)

• Hardware: Tested on a physical SO-101 follower arm with STS3215 motors.
• Manual Verification:
  1. Ran lerobot-calibrate and set the wrist to a specific orientation.
  2. Checked the generated calibration.json.
  3. Confirmed that range_min and range_max are symmetric around the set position (e.g., current_pos=2028 resulted in min=0, max=4056).
  4. Verified that the wrist no longer "jumps" or offsets when teleoperation starts.

Bash

# To reproduce:
lerobot-calibrate --robot.type=so101_follower --robot.id=test_arm

Checklist (required before merge)

• Linting/formatting run (pre-commit run -a) - Note: If you haven't run this, you can check if CI passes.
• All tests pass locally (pytest)
• Documentation updated - N/A
• CI is green

Reviewer notes

The core logic uses safe_delta to prevent the range from exceeding the physical limits (0-4095) while maintaining symmetry. This is crucial because motors_bus.py uses the arithmetic mean of the range to define the normalized zero point.