v0.27.0: detect AACS-scrambled units by raw TS sync, not flag bits

Rename is_unit_encrypted -> is_aacs_scrambled and decide encryption from the unit's MPEG-TS sync bytes (destroyed by the encrypted body) instead of the TP_extra copy-control (byte 0) or TS scrambling-control (byte 7) flags, which discs do not set reliably. One shared predicate now backs the decrypt gate and out-of-band key validation, so callers agree on what 'encrypted' means. Decryption restores the syncs, so a decrypted unit reads as clear and there is no flag to clear.

src/aacs/keys.rs

@@ -1292,7 +1292,7 @@ mod tests {
         let original = std::fs::read(unit_path).unwrap();
         assert_eq!(original.len(), ALIGNED_UNIT_LEN);
         assert!(
-            super::super::decrypt::is_unit_encrypted(&original),
+            super::super::decrypt::is_aacs_scrambled(&original),
             "Unit should be encrypted"
         );