PES pipeline: InputStream on DiscStream + IsoStream, MkvOutputStream

- DiscStream: next_frame() reads sectors → decrypts → demuxes → returns PesFrame
- IsoStream: same pattern, reads from ISO file
- MkvOutputStream: accepts PesFrame, writes MKV via MkvMuxer
- InputStream trait: next_frame(), info(), codec_private(), headers_ready()
- OutputStream trait: write_frame(), finish()
- FileSectorReader: SectorReader backed by a file
- PesFrame: track + pts + keyframe + data

Old Read/Write IOStream impls preserved for backward compatibility.
Next: replace pipe() in CLI to use PES pipeline.

src/mux/mod.rs

@@ -28,6 +28,7 @@ pub mod lookahead;
 mod m2ts;
 pub mod meta;
 pub mod mkv;
+pub mod mkvout;
 mod mkvstream;
 pub mod network;
 pub mod null;