Relocate the shared MKB machinery into a single mkb module: the record
framing walker + MkbRecord view (from variant), the MkbType/AacsVersion
classification, the MKB-file utilities, and the record-body finders (from
keys). Fixes the inversion where the MKB parser lived in the 2.1-only
variant module. variant.rs keeps its local MkbRecord-based mkb_find_mk_dv
(name collision with the raw one; unified in the dedup follow-up).
Relocation only. Proven byte-identical to the pre-refactor state via the
function-body logic hash (imports normalized out); 2210 tests green.
Matthew Jackson
f55f11d043