A managed set of keys, each with exactly one owner in the cluster at any given time. When an owner's lease is lost, ownership moves to another node. Runs one portunus_election per key. The callback module supplies the key set and the per-key work:

-callback keys(Args :: term()) -> [Key :: term()].
-callback start(Key :: term(), Token :: portunus:token(), Args :: term()) ->
    {ok, State :: term()}.
-callback stop(Key :: term(), State :: term()) -> ok.

start/3 must link whatever it starts to the calling process: a crashed election loses the callback state, and an unlinked resource then keeps running with no way to stop it.

Every node contends eagerly, and the election keeps a single winner per key until that winner's lease is lost. Pass #{affinity => Spec} (see portunus_affinity) to steer which node wins, for example a preferred-owner or consistent-hash hint; the default is FIFO in arrival (registration) order.