Feature/2154 stepper motor update

[JulianHammerl]

• Tickets addressed: xmera-2154
• Review: By commit
• Merge strategy: Merge (no squash)

Description

Updates stepperMotor so it stays consistent with stepperMotorController under
interrupts and stop commands, and exposes absolute position + motion status on its
output.

Five commits, intended to be reviewed in order:

1. Anchor each step's tInit to the previous step's tf so the motor rate doesn't
   drift when the update period doesn't divide stepTime evenly.
2. Add isMotorMoving to the output message.
3. Add motorPosition (cumulative net signed step count, never reset) to the output
   message.
4. Add stopMotorCommand to MotorStepCommandMsgPayload; when set, halt at the next
   step boundary via a pendingStop flag.
5. Refresh stepperMotor.rst.

Verification

All tests still pass.

Documentation

Documentation updated.

Future work

None