Behaviour for heartbeat source adapters.

A source is any process that calls PhiAccrual.observe/2 in response to evidence that a remote node is alive. The evidence may be an explicit ping, a GenServer reply, a :pg broadcast, a :global sync — anything that arrives from the remote node on the local BEAM.

v1 ships one reference source: PhiAccrual.Source.DistributionPing. Applications are free to write their own — the behaviour is minimal because sources are ordinary supervised processes that happen to call into the detector.

Clock discipline. A source MUST pass timestamps from PhiAccrual.Clock.now/0 (or :erlang.monotonic_time(:millisecond), equivalently). Cross-node timestamps are meaningless here and must never appear in observe/2 calls.