Populate LowState tick in the Python interface

[tomasz-lewicki]

Problem

PyLowState.tick is always 0, no matter what the robot is publishing. The
field exists on the struct and is exposed to Python, but it is never filled in:

• ProcessLowState (both the HG and GO2 overloads) copies motor_state,
  imu_state, and mode_machine out of the incoming DDS LowState_, but never
  reads low_state.tick().
• ConvertToPyLowState sets py_state.mode_machine but never py_state.tick.

So tick stays at its default 0 while every other field updates correctly —
making it unusable for timing / staleness / drop-detection on the reader side.

Fix

Carry the tick through, mirroring the existing mode_machine_ passthrough:

• add a tick_ member,
• set it in both ProcessLowState overloads,
• surface it in ConvertToPyLowState.

Four lines of real change; no API or behavior change beyond the field now being
populated. tick_ is a 32-bit aligned scalar written from the DDS callback
thread and read from the caller thread — the same benign lock-free pattern
already used for mode_machine_.

Testing

Built the extension and read a live G1 low-state stream over a network
interface. Before: tick constant at 0. After: tick is monotonic and
increments on every read_low_state() (IMU and joint fields unchanged, as
expected).

Note

Does not bump the package version — leaving the release/wheel bump to
maintainers.

~Claude