Kept recoverable rather than maintained. The design carried over verbatim; what
did not carry over is compile-time enforcement of the invariants (no Tier::min,
exhaustive match on Reversibility), which the Python port reconstructs with
mypy --strict and typing.assert_never.

Worth keeping for one reason: if detent ever needs to run in a Rust userland,
this is a working, tested starting point for a design that will by then have been
validated.