Require a pack start code before descrambling a sector

css::is_scrambled reads bits 4-5 of byte 0x14 and nothing else. That is a
sound test once a caller has committed to a title's VOB data, where every
sector is an MPEG-2 PS pack and byte 0x14 always means what it says.
descramble_region is not such a caller: it is handed arbitrary regions of a
disc, so it also sees IFO, UDF and ISO 9660 sectors — raw structures where
byte 0x14 is whatever that format happens to store there.

Measured on a real disc: the second sector of VIDEO_TS.IFO holds 0x15 at
offset 0x14 while starting 00 26 00 00, which is not a pack. The flag test
read it as scrambled, descrambled it, and destroyed 1912 of its 2048 bytes.
That sector carries TT_SRPT, so the title table went with it — the disc
enumerated 38 titles and an image decrypted from it enumerated 10, silently,
at exit 0.

is_scrambled_pack already existed with the right predicate. Use it here. It
costs nothing: a genuinely scrambled VOB sector always carries the pack start
code, and no IFO sector does.

Verified end to end — the decrypted image's `info` output is now identical to
the source disc's, 38 titles both, differing only in the CSS: Encrypted line.

The fixtures moved with it. Four of them built a sector by setting byte 0x14
alone, which no real scrambled sector looks like; they now build packs.

src/decrypt.rs

@@ -987,6 +987,12 @@ mod tests {
     /// scramble flag.
     fn make_css_sector(title_key: &[u8; 5], seed: &[u8; 5], body_fill: u8) -> (Vec<u8>, Vec<u8>) {
         let mut sector = vec![body_fill; 2048];
+        // A real scrambled DVD sector is an MPEG-2 PS pack, so it begins with
+        // the pack start code. The descrambler requires it before trusting
+        // byte 0x14 — without it this fixture is a sector shape that cannot
+        // occur on a disc, and the test would pass while the production gate
+        // rejected every sector like it.
+        sector[0x00..0x04].copy_from_slice(&[0x00, 0x00, 0x01, 0xBA]);
         sector[0x14] = 0x30; // scramble flag (bits 4-5)
         sector[0x54..0x59].copy_from_slice(seed);
         let plaintext = sector.clone();
@@ -1064,6 +1070,9 @@ mod tests {
         period: usize,
     ) -> (Vec<u8>, Vec<u8>) {
         let mut plaintext = vec![0u8; 2048];
+        // Real scrambled DVD sectors are MPEG-2 PS packs; the scramble policy
+        // requires the pack start code as well as the flag bits.
+        plaintext[0x00..0x04].copy_from_slice(&[0x00, 0x00, 0x01, 0xBA]);
         plaintext[0x14] = 0x10; // scramble flag
         // Periodic run from 0x59 (just above the seed) through 0x80 and on into
         // the encrypted region; phase anchored to offset 0 so it is continuous