Type Alias Watermarks 

pub type Watermarks = [u64; 3];

One past the last published sequence, per signal, indexed by Signal::index. Assembled from the block directory listing at boot; see the module docs.

Exclusive rather than inclusive on purpose. An inclusive “highest published sequence” has no value that means nothing published: zero is a real sequence, so the empty state and “sequence 0 is durable” are the same number, and the first export after every restart is silently dropped from the replay. Exclusive makes the empty state 0 and needs no sentinel.