Audit fixes + DVD support foundation (IFO, PS demux, MPEG-2, CSS crack)

Audit fixes (14 critical, 22 warnings):
- UDF: bounds checks on all ICB/FID parsing from disc data
- SCSI Linux: saturating_sub on residual, CDB length guard, buffer size guard
- SCSI macOS: SCSITaskStatus u32 (was u8 — stack corruption)
- AACS: EC mod_inv returns infinity instead of panic, key reduced mod n
- AACS: do_handshake tries all host certs (was returning on first failure)
- H.264: bounds check on SPS < 4 bytes
- ContentReader: error on missing unit key (was zero-fill)
- KEYDB: flat redirect loop (was recursive), 100MB response limit, Windows HOME fallback
- ISO writer: AVDP extent order, partition length, allocation cap
- Network: removed TCP_NODELAY on bulk stream
- MKV: guard on u64::MAX seek
- disc.rs: saturating_sub on extent offset, simplified dead region code
- cargo fmt (610 violations), cargo clippy --fix (55 auto-fixes)

DVD support (new files):
- src/ifo.rs — IFO parser (VIDEO_TS.IFO, VTS_XX_0.IFO, PGC chains, cells, streams) — 13 tests
- src/mux/ps.rs — MPEG-2 Program Stream demuxer (pack headers, PES, private stream 1) — 12 tests
- src/mux/codec/mpeg2.rs — MPEG-2 video parser (sequence headers, I-frame detection) — 15 tests
- src/css/crack.rs — split-attack algorithm (LFSR cipher needs verification — test ignored)

226 tests total (was 186), 1 ignored (CSS crack needs cipher verification).

src/css/mod.rs

@@ -0,0 +1,62 @@
+//! CSS (Content Scramble System) — DVD disc encryption.
+//!
+//! CSS uses a weak 40-bit LFSR stream cipher (broken since 1999).
+//! No keys needed — the title key is cracked from encrypted content
+//! using a known-plaintext attack on MPEG-2 PES headers.
+//!
+//! Usage:
+//! ```rust,ignore
+//! let key = css::crack_key(reader, &extents)?;
+//! css::descramble_sector(&key, &mut sector);
+//! ```
+
+pub mod crack;
+pub mod lfsr;
+
+use crate::disc::Extent;
+use crate::sector::SectorReader;
+
+/// CSS decryption state for a DVD title.
+#[derive(Debug, Clone)]
+pub struct CssState {
+    /// Cracked 5-byte title key
+    pub title_key: [u8; 5],
+}
+
+/// Crack the CSS title key by reading encrypted sectors and applying
+/// a known-plaintext attack on MPEG-2 headers.
+///
+/// Reads a few sectors from the first extent, finds one with the
+/// scramble flag set, and cracks the key.
+pub fn crack_key(reader: &mut dyn SectorReader, extents: &[Extent]) -> Option<CssState> {
+    if extents.is_empty() {
+        return None;
+    }
+
+    let ext = &extents[0];
+    let mut sectors = Vec::new();
+
+    // Read first 10 sectors from the main extent
+    let count = ext.sector_count.min(10);
+    for i in 0..count {
+        let mut buf = vec![0u8; 2048];
+        if reader.read_sectors(ext.start_lba + i, 1, &mut buf).is_ok() {
+            sectors.push(buf);
+        }
+    }
+
+    // Try cracking from the collected sectors
+    let key = crack::crack_from_sectors(&sectors)?;
+
+    Some(CssState { title_key: key })
+}
+
+/// Descramble a single CSS-encrypted sector in place.
+pub fn descramble_sector(state: &CssState, sector: &mut [u8]) {
+    lfsr::descramble_sector(&state.title_key, sector);
+}
+
+/// Check if a sector has the CSS scramble flag set.
+pub fn is_scrambled(sector: &[u8]) -> bool {
+    sector.len() >= 2048 && (sector[0x14] >> 4) & 0x03 != 0
+}