Central round-trippable codec: parse + to_keydb_cfg in one place.
Emits HC, DK, PK, then disc entries sorted by hash. Round-trip test
(parse -> serialize -> re-parse) covers HC/DK/PK/disc. disc_id -> vid
names the field for what it is (the I-field volume ID).
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.