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/decrypt.rs

@@ -54,13 +54,7 @@ pub fn decrypt_sectors(
             let uk = match unit_keys.get(unit_key_idx) {
                 Some((_, k)) => *k,
                 None => {
-                    return Err(crate::error::Error::DecryptFailed {
-                        reason: format!(
-                            "unit_key_idx {} out of range (have {} keys)",
-                            unit_key_idx,
-                            unit_keys.len()
-                        ),
-                    });
+                    return Err(crate::error::Error::DecryptFailed);
                 }
             };
             let rdk = read_data_key.as_ref();