Remove Seek/File dependencies from stream readers

Streams are streams — they take impl Read, not Read+Seek or File.

- MkvStream::open takes impl Read (was Read+Seek)
  - EBML element skipping uses skip_bytes() instead of seek(Current)
  - Byte position tracking uses remaining-bytes counter, not stream_position()
  - Removed file_size from open (progress is CLI concern)
- M2tsStream::open takes impl Read (was Read+Seek)
  - Buffers first 1MB for FMKV header / PMT scan
  - Uses chain reader (buffered head + rest) for sequential reading
  - Duration unknown without seeking (0.0) — CLI can set from metadata
- Removed ReadSeek trait (no longer needed)
- WriteSeek kept (MKV muxer container format requires seeking internally)

src/mux/mod.rs

@@ -75,9 +75,6 @@ pub trait IOStream: Read + Write {
     }
 }
 
-// Combined traits for internal trait objects.
-pub(crate) trait ReadSeek: Read + Seek {}
-impl<T: Read + Seek> ReadSeek for T {}
-
+// WriteSeek — used internally by MKV muxer (container format requires seeking).
 pub trait WriteSeek: Write + Seek {}
 impl<T: Write + Seek> WriteSeek for T {}