feat: add Leadshine DM2J stepper motor driver

[ZiWei09]

Summary

• Add Leadshine (雷赛) DM2J series integrated stepper motor driver (unilabos/devices/motor/leadshine_dm2j.py)
• Add motor device registry YAML with DM2J and ZDT X42 definitions (unilabos/registry/devices/motor.yaml)
• RS485 Modbus RTU communication via pymodbus, supporting position control (PR immediate trigger), speed control (JOG), homing, enable/disable, position read/clear, and alarm management

Details

The DM2J driver supports models: DM2J-RS522 / RS542 / RS556 / RS570

Key design decisions:

• Position control: Uses PR path 0 immediate trigger mode (write 8 registers to 0x6200-0x6207) for maximum flexibility
• Speed control: Uses RS485 JOG commands with a background keepalive thread (40ms interval, manual requires <50ms)
• Serial defaults: Even parity + 2 stop bits to match factory configuration (configurable)
• Thread safety: All Modbus R/W protected by RLock; background polling for status/position

Test plan

• Verify Python syntax: python -c "import ast; ast.parse(open('unilabos/devices/motor/leadshine_dm2j.py').read())"
• Verify YAML: python -c "import yaml; yaml.safe_load(open('unilabos/registry/devices/motor.yaml'))"
• Hardware test with DM2J-RS542: run python unilabos/devices/motor/leadshine_dm2j.py
• Verify Uni-Lab-OS integration: configure experiment with motor.leadshine_dm2j device type

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