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.

tests/pass_n_patch_fix.rs

@@ -42,6 +42,10 @@ fn css_decrypt_of_an_uncrackable_sector_still_descrambles() {
     // unit key either opens a unit or does not, whereas a CSS title key is
     // recovered from data whose recoverability varies sector by sector.
     let mut sector = vec![0xFFu8; 2048];
+    // Scrambled DVD sectors are MPEG-2 PS packs. The descramble policy requires
+    // the pack start code as well as the flag bits, because byte 0x14 means
+    // something else entirely in an IFO, UDF or ISO 9660 sector.
+    sector[0x00..0x04].copy_from_slice(&[0x00, 0x00, 0x01, 0xBA]);
     sector[0x14] |= 0x30; // CSS scramble flag, bits 4-5
 
     let title_key: [u8; 5] = [0x42, 0x13, 0x37, 0xBE, 0xEF];