v2.0.1

seesaw-tgg 2.0.1

Three defects from a review of 2.0.0, all in shipped code:

1. The identity encoding was ambiguous — two adjacent variable-length
   fields with no boundary marker. Every identity changes between 2.0.0
   and 2.0.1.
2. Retraction stopped at a materialization: a folded product was left
   standing and a delta produced no tombstone.
3. A rank ceiling was void whenever the runtime mask was set.

Rust 156 tests green here, Java 69; in the development tree 342 and 111.