Add smooth PreciseFlex Cartesian route primitive

[j0nch]

Summary

• Add a PreciseFlex-specific move_through_cartesian_poses backend method for planned, blended Cartesian pose routes.
• Plan IK from one initial state snapshot so taught transit waypoints do not require end-of-motion/current-position reads between each pose.
• Temporarily set the active motion profile InRange to -1 for blending, then restore the original profile after the final waypoint.

Notes

This intentionally stays PF400-specific rather than adding a generic arm capability, since the behavior depends on PreciseFlex IK state, motion-profile blending, and queued moveJ behavior.

Validated downstream on two Retro workcells using Cartesian teachpoints before opening this PR.

Tests

• python3 -m unittest pylabrobot.brooks.precise_flex.tests.arm_backend_tests