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/keysource.rs

@@ -563,10 +563,7 @@ mod tests {
     struct HasKey([u8; 16]);
     impl KeySource for HasKey {
         fn get_uk(&self, _ctx: &dyn ResolveCtx) -> Result<Vec<UnitKey>, Error> {
-            Ok(vec![UnitKey {
-                idx: 0,
-                key: self.0,
-            }])
+            Ok(vec![UnitKey::new(0, self.0)])
         }
     }
 
@@ -612,10 +609,7 @@ mod tests {
                 if let Ok(s) = ctx.samples(8) {
                     self.seen.lock().unwrap().extend(s);
                 }
-                Ok(vec![UnitKey {
-                    idx: 0,
-                    key: self.key,
-                }])
+                Ok(vec![UnitKey::new(0, self.key)])
             }
         }