TidGenerator.withClockId constructor
Returns a generator with an explicit clockId and an optional now,
both injectable so tests are deterministic.
clockId is masked to _clockIdBits bits, so an out-of-range value
cannot corrupt the timestamp. now returns microseconds since the UNIX
epoch and defaults to the system clock.
Implementation
TidGenerator.withClockId(final int clockId, {final int Function()? now})
: _clockId = clockId & ((1 << _clockIdBits) - 1),
_now = now ?? _systemNow;