v0.2.0 — harden envelope, opaque CipherError, stats API, recovery refactor

Breaking release. The headline change is the envelope hardening (false-positive rate on legacy encrypted files: 2⁻³² → 2⁻⁵⁶). Also adds stats()/BufferStats/len/is_empty, makes CipherError opaque with source() chaining, and refactors recover() so the mutex is no longer held across filesystem metadata calls.

See CHANGELOG for the full change history.