v0.10.1: Streams are PES, Disc::copy() for sector dumps, zero English

Architecture:
- One stream per format, bidirectional PES (read/write on same type)
- IsoStream merged into DiscStream (one type, any SectorReader)
- Disc::copy() for disc→ISO raw sector dump
- IOStream trait deleted, all byte-level Read/Write removed
- ContentReader/OpenDisc/open_title/open_input/open_output deleted
- CountingStream wrapper for progress tracking

Error codes:
- All io::Error English strings replaced with Error enum variants
- From<Error> for io::Error conversion
- Unused variants removed, new stream/mux variants added

Deleted: mkvout.rs, pesout.rs, isowriter.rs, mkv-muxer-plan.md
Updated: all docs, README stream table, CHANGELOG

238 tests, 0 clippy warnings.

src/mux/ebml.rs

@@ -171,10 +171,7 @@ pub fn read_id(r: &mut impl Read) -> io::Result<(u32, usize)> {
             4,
         ))
     } else {
-        Err(io::Error::new(
-            io::ErrorKind::InvalidData,
-            "invalid EBML ID",
-        ))
+        Err(crate::error::Error::MkvInvalid.into())
     }
 }
 
@@ -325,10 +322,7 @@ pub fn read_vint(r: &mut impl Read) -> io::Result<(u64, usize)> {
         r.read_exact(&mut b)?;
         return Ok(((((b0 & 0x3F) as u64) << 8) | b[0] as u64, 2));
     }
-    Err(io::Error::new(
-        io::ErrorKind::InvalidData,
-        "unsupported VINT width",
-    ))
+    Err(crate::error::Error::MkvInvalid.into())
 }
 
 // ============================================================