ot::Mac::RxFrame::GetTimestamp() const method

Returns the timestamp when the frame was received. The value SHALL be the time of the local radio clock in microseconds when the end of the SFD (or equivalently: the start of the first symbol of the PHR) was present at the local antenna, see the definition of a "symbol boundary" in IEEE 802.15.4-2020, section 6.5.2 or equivalently the RMARKER definition in section 6.9.1 (albeit both unrelated to OT). The time is relative to the local radio clock as defined by `Radio::GetNow()`.