unit_keys_from now resolves the Media Key in order: stored per-disc MK -> keydb Processing Key pool (mk_from_pk vs this disc's own MKB) -> device-key pool (mk_from_dk), then MK+VID -> VUK -> UK. MK/VUK entries still honored directly; cross-disc MK-pool brute stays retired. Fixes the factually-wrong justifying comment + adds PK-pool KATs.