Unify gripper API: move_gripper(width, force_sensing) + class hierarchy refactor

[rickwierenga]

Summary

• Collapses open_gripper(width) / close_gripper(width) into a single backend primitive move_gripper(width, force_sensing=False) and adds required min_gripper_width / max_gripper_width (Optional[float]) properties on CanGrip. Widths are always in mm.

• Frontend open_gripper() / close_gripper() become no-width wrappers that read max/min from the backend and raise NotImplementedError when the corresponding limit is None. They still accept backend_params.

• Splits the arm class hierarchy so GripperArm is the abstract base that owns gripper-width controls, with three concrete subclasses:

  _BaseArm
    └── GripperArm (abstract; owns gripper-width controls)
          ├── FixedAxisGripperArm  (was GripperArm; grip_axis="x"|"y")
          ├── OrientableGripperArm (was OrientableArm)
          └── ArticulatedGripperArm (was ArticulatedArm)
  

• Concrete backends migrated:

  • iSWAP: GF/GC commands flow through move_gripper; CloseGripperParams renamed to GripParams; placeholder min=50 / max=145 mm.
  • CoreGripper: min=9, max=None; move_gripper(_, force_sensing=False) still fires ZO, force-sensing raises.
  • PreciseFlex PF400: placeholder min=60 / max=145 mm; routes to firmware gripper 1 / gripper 2.
  • xArm6: min/max_gripper_width delegate to existing gripper_min_mm / gripper_max_mm.

• Deletes the legacy pylabrobot.arms shim package entirely.

• Updates tests, notebooks, and architecture / user-guide docs.

Notes for review

• iSWAP / PF400 min/max are placeholders — please overwrite with verified hardware limits.
• PF400's GripOpenPos / GripClosePos firmware commands may want a closed_gripper_position-anchored mm→units helper (called out in the CanGrip docstring); current behaviour pass-through preserved.
• xArm6 backend tests have 3 pre-existing failures on v1b1 that this branch does not fix or worsen (test_custom_mm_per_gripper_unit, test_pick_up_at_joint_position, test_pick_up_at_location_move_then_close).

Test plan

• python -m pytest pylabrobot/capabilities/arms pylabrobot/hamilton/liquid_handlers/star/tests pylabrobot/ufactory/xarm6/backend_tests.py (58 pass; 3 pre-existing xArm6 failures unchanged)
• Manually exercise iSWAP open_gripper() / close_gripper() / move_gripper(..., force_sensing=True, backend_params=GripParams(...)) on real hardware
• Manually exercise PF400 open/close and verify firmware accepts mm widths (otherwise wire the mm→units helper)
• Rebuild docs and skim updated arms.md / iswap.ipynb / xarm6 hello-world / precise_flex hello-world

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