Reading Timestamp should reference a cached offset

[Poofjunior]

There is a race condition where it is possible for the reported Timestamp seconds and microseconds to reference a different offset and therefore report an erroneous value. This happens if the timestamp offset is updated via interrupt while the timestamp is being added into the reply message.

The fix would be to make sure we reference the same offset when we retrieve the time and compute it to update the timestamp registers and then read them. It really just boils down to making sure that the local offset value (offset_us_64_) is either a monotonic read or read from a double-buffer.
https://github.com/AllenNeuralDynamics/harp.core.rp2040/blob/main/firmware/inc/harp_core.h#L351