Expose STARBackend.measure_iswap_gripper_force()

[BioCam]

• Adds STARBackend.measure_iswap_gripper_force() -> float, returning the force currently measured by the iSWAP gripper in Newtons.
• Wraps the previously unexposed R0 QH firmware command.
• Enables closed-loop grip verification, grip-slip detection, and adaptive grip-strength tuning — none of which were previously possible in PLR.

Implementation notes

• Firmware reports force in 1/10 N units; wrapper converts to N. Effective range is 0–20.0 N, with 16.0 N being the max settable grip via gd.
• round(x, 1) caps at the firmware's true resolution and gives clean repr() output. Matches existing PLR convention for unit conversions.
• RuntimeError if iSWAP isn't installed, matching the existing R0 query wrappers (request_iswap_position, iswap_rotation_drive_request_y, request_iswap_initialization_status).
• Naming follows a measure_* convention for live sensor readings, distinct from request_* used for state queries (position, init-status). New convention; happy to discuss.

Test plan

• Static: existing CI (lint, type-check) passes.
• On a real STAR with iSWAP installed:
  • Call with iSWAP idle / open → expect ~0 N
  • Call while gripping a plate at default strength (gw=5) → expect ~5–10 N
  • Call with gw=9 → expect closer to 16 N
  • Confirm response width matches fmt="qh###"; if firmware sends a different number of digits, adjust the format string.