Fix all clippy warnings: dead code, match patterns, type complexity, docs

- Remove unused pes_buf field from M2tsStream and unused TS_PACKET/BD_TS_PACKET constants
- Replace match-with-single-pattern with if let (3 instances in drive/mod.rs)
- Replace match-can-be-? with ? operator for scsi::open call
- Add type aliases PesSetup and MkvHeaderResult to reduce type complexity
- Collapse identical if/else branches in tsmux.rs build_pes_header
- Use RangeInclusive::contains instead of manual range checks
- Make WriteSeek trait pub (was pub(crate) but leaked through pub fn)
- Remove empty line after doc comment in disc.rs
- Fix doc list item indentation in scsi/linux.rs (12 instances)

src/mux/mod.rs

@@ -79,5 +79,5 @@ pub trait IOStream: Read + Write {
 pub(crate) trait ReadSeek: Read + Seek {}
 impl<T: Read + Seek> ReadSeek for T {}
 
-pub(crate) trait WriteSeek: Write + Seek {}
+pub trait WriteSeek: Write + Seek {}
 impl<T: Write + Seek> WriteSeek for T {}