Timeframe::floor computed `ts - ts.rem_euclid(bucket)`. For a timestamp
within one bucket of i64::MIN the subtrahend is a positive remainder
and the true boundary lies below i64::MIN, so the subtraction overflowed
and panicked in debug builds.
Switch to saturating_sub: the result clamps to i64::MIN in that
practically unreachable case and stays exact everywhere else. floor
keeps its infallible `-> i64` signature, so neither push path changes.
kingchenc
f33f59ad68