Add `aacs::content::unit_key_opens(unit, key, format) -> bool` behind a new
non-default `keyproof` feature — one bit (key right/wrong), never plaintext —
for a private offline key-resolver (kdb). BD-TS per-packet fast reject, with a
decrypt_unit + is_clean fallback for PS/odd units. A not(keyproof)-gated
compile_fail door in keys/public_api_cannot_decrypt_aacs pins its absence by
default, so the default public API still cannot decrypt AACS (KU §2.2). Also
expose test_util::encrypt_unit alongside decrypt_unit for consumer fixtures.
Equivalence test pins the fast path to decrypt_unit + is_clean. Version left to
the release owner.
Matthew Jackson