1.3.2: AACS 2.1 FMTS variant-decode foundation

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.

src/aacs/derive.rs

@@ -612,10 +612,7 @@ mod resolve_candidate_tests {
     /// A bare UK candidate is terminal — it returns itself keyed by its own idx.
     #[test]
     fn resolve_candidate_uk_is_itself() {
-        let uk = UnitKey {
-            idx: 2,
-            key: [0x9u8; 16],
-        };
+        let uk = UnitKey::new(2, [0x9u8; 16]);
         let r = resolve_candidate(&KeyCandidate::Uk(uk), &[], &[], None).expect("uk is terminal");
         assert_eq!(r.unit_keys, vec![(2, uk.key)]);
         assert!(r.vuk.is_none() && r.mk.is_none());