Clock discipline for the φ-accrual detector.

The detector reasons ONLY about local arrival times — every timestamp passed into PhiAccrual.observe/2 MUST come from the same monotonic clock running on the node hosting the detector. Cross-node timestamps are meaningless here and must never appear in interval calculations.

now/0 returns integer milliseconds from :erlang.monotonic_time/1. Estimators accept an alternate clock_fn at startup for deterministic testing — production code should not call alternate clocks.