Fix audit v2 criticals: DiscStream read loop, CSS crack, ISO writer

Critical fixes:
- DiscStream: persistent read state (was creating new ContentReader per call)
  Full error recovery, AACS/CSS decryption, extent tracking across reads
- CSS crack: labeled 'outer continue (was targeting wrong loop)
- CSS crack: LFSR0 polynomial fixed to match cipher (shifts 8,1,3,7)
- CSS lfsr: operator precedence clarified in LFSR0 init

Warning fixes:
- ISO writer: UDF tag checksums computed (was zeros)
- DVD extents: saturating_add for overflow safety
- Removed dead fields: HandshakeResult.error, ContentReader.content_format
- Added TODO for ISO long_ad >4GB support

319 tests, 20 clippy warnings remaining.

src/mux/disc.rs

@@ -10,7 +10,7 @@
 use super::IOStream;
 use crate::disc::{
     ContentFormat, Disc, DiscTitle, Extent,
-    DEFAULT_BATCH_SECTORS, MIN_BATCH_SECTORS, RAMP_BATCH_AFTER, RAMP_SPEED_AFTER,
+    MIN_BATCH_SECTORS, RAMP_BATCH_AFTER, RAMP_SPEED_AFTER,
     SLOW_SPEED_AFTER, detect_max_batch_sectors,
 };
 use crate::drive::DriveSession;
@@ -54,6 +54,7 @@ pub struct DiscStream {
     extents: Vec<Extent>,
     current_extent: usize,
     current_offset: u32,
+    #[allow(dead_code)]
     content_format: ContentFormat,
     aacs: Option<AacsDecrypt>,
     css: Option<crate::css::CssState>,
@@ -349,8 +350,7 @@ impl Write for DiscStream {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
-    use crate::disc::{ContentFormat, DiscTitle, Extent};
+    use crate::disc::Extent;
 
     /// Build a minimal DiscStream with fake extents for testing state advancement.
     /// We cannot call `DiscStream::open()` without a real drive, so we construct