Add Hettich robotic centrifuge driver

[rickwierenga]

Summary

• add a Generation 2 Hettich serial driver with an abstract protocol base and concrete MIKRO 220, ROTANTA 460, and ROTINA 380 model classes
• provide state-based hatch control, rotor positioning, stored-program recall, emergency stop, exact target-speed spin duration, and programmatic RPM/RCF conversion for MIKRO rotors 2334 and 2394
• validate exact device identity codes and expose chamber temperature only on refrigerated models
• add mocked protocol coverage, API documentation, a complete MIKRO 220 hello-world notebook, and the full device-registry entry

Validation

• python -m pytest pylabrobot/hettich/centrifuge_tests.py -q — 36 passed
• python -m mypy pylabrobot/hettich/ — passed
• python -m ruff check pylabrobot/hettich — passed
• python -m pytest docs/_exts/plr_devices/registry_tests.py -q — 26 passed, 404 subtests passed
• notebook JSON, cell IDs, cleared outputs, and Python syntax validated
• make docs-fast — passed with warnings treated as errors

Hardware

Setup, identity, status, hatch control, slow rotor positioning, and centrifugation were exercised on a MIKRO 220 POS running firmware 01.21. The final target-speed duration accounting and duration-limit preflight are additionally covered by mocked protocol tests.