Fix MPEG-2 PS demuxer: codec parsers, sequence header extraction, descramble

- Route DVD PS packets through codec parsers (was bypassing them)
- Extract MPEG-2 sequence header with quantizer matrices for MKV codec_private
- Calculate exact sequence header size from intra/non-intra matrix flags
- Capture sequence extension (B5) from subsequent PES packets
- Revert TAB1 permutation in descramble (XOR-only is correct)
- Fix CSS roundtrip tests for new descramble behavior

src/css/lfsr.rs

@@ -67,7 +67,7 @@ pub fn descramble_sector(title_key: &[u8; 5], sector: &mut [u8]) {
         lfsr0 = (lfsr0 >> 8) | ((o_lfsr0 as u32) << 24);
 
         combined += TAB5[o_lfsr1 as usize] as u32 + TAB4[o_lfsr0 as usize] as u32;
-        *byte = TAB1[*byte as usize] ^ (combined & 0xFF) as u8;
+        *byte ^= (combined & 0xFF) as u8;
         combined >>= 8;
     }