Replace that POS defrag, update Algo names, fix tests, rework initial

algo.

src/mapping/cluster.rs

@@ -3,6 +3,8 @@ use super::{Domain, Stage};
 use serde::{Deserialize, Serialize};
 
 /// A map for data stored in memory for processing and saving to disk.
+// derived Ord impl *should* use self.domain.start to sort? Not sure.
+// Use `sort_by_key()` to be safe.
 #[derive(Clone, Copy, Debug, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Cluster {
     pub domain: Domain,