- mkb_content_len walks the MKB record stream and returns where it ends,
so callers can drop the trailing zero padding. MKB files are allocated
to a fixed size (~128 MiB) with records at the front; both MKB_RO and
MKB_RW can be padded. read_aacs_inputs* now trim to the real records
(~few MB) instead of shipping ~128 MiB of zeros.
- decrypt_keys() returns None when unit_keys is empty: an AACS state that
carries only a VID (out-of-band resolution, pre-key) is 'encrypted, no
keys', not a usable empty key set.