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/meta.rs

@@ -247,7 +247,7 @@ pub fn read_header(r: &mut impl Read) -> io::Result<Option<M2tsMeta>> {
     r.read_exact(&mut len_buf)?;
     let json_len = u32::from_be_bytes(len_buf) as usize;
     if json_len > MAX_JSON_SIZE {
-        return Err(io::Error::new(io::ErrorKind::InvalidData, "FMKV JSON too large"));
+        return Err(crate::error::Error::NoMetadata.into());
     }
 
     let mut json_buf = vec![0u8; json_len];