Move codec_privates onto DiscTitle, eliminate duplicate methods

Design fix: codec_privates are now a field on DiscTitle, not a separate
parameter passed through the pipeline. This eliminates the root cause of
the network codec_private bug (forgot to pass the separate param).

API changes:
- output() takes (url, &DiscTitle) — no separate codec_privates param
- MkvOutputStream::create, M2tsOutputStream::create, NetworkOutputStream::connect
  all read codec_privates from title.codec_privates
- M2tsMeta::from_title() takes only &DiscTitle — reads privates from title
- Deleted from_title_with_privates (was the wrong-name duplicate)
- Merged read_header + read_header_from_stream into one read_header(impl Read)
- Deleted finish(self) from TsMuxer, keep only finish(&mut self)

Rule: ONE public method per action. No _with_X, _from_Y, _ref variants.

src/mux/m2ts.rs

@@ -90,7 +90,8 @@ impl M2tsStream {
         let file_size = reader.seek(SeekFrom::End(0))?;
         reader.seek(SeekFrom::Start(0))?;
 
-        // Try FMKV metadata header
+        // Try FMKV metadata header — save position so we can seek back on failure
+        let start = reader.stream_position()?;
         if let Ok(Some(m)) = meta::read_header(&mut reader) {
             let header_end = reader.stream_position()?;
             let content_size = file_size.saturating_sub(header_end);
@@ -113,8 +114,8 @@ impl M2tsStream {
             });
         }
 
-        // Fallback: scan PMT for streams, PTS for duration
-        reader.seek(SeekFrom::Start(0))?;
+        // No FMKV header — seek back and try PMT scan
+        reader.seek(SeekFrom::Start(start))?;
         let mut buf = vec![0u8; SCAN_SIZE];
         let n = reader.read(&mut buf)?;