Add UnitKey.variant_number (0 = ordinary, 1..32 = forensic variant) with new/variant constructors, and aacs::variant_select — resolve a disc's single variant and classify each aligned unit (default / variant / drop foreign / conceal keyless). Correct IndividualSegment.tbl: the per-record field is the variant (cycles 1..32 on a retail disc), not a segment number — Segment.number -> Segment.variant.
