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/disc/mod.rs

@@ -116,6 +116,9 @@ pub struct DiscTitle {
     pub extents: Vec<Extent>,
     /// Content format for this title
     pub content_format: ContentFormat,
+    /// Codec initialization data per stream (SPS/PPS, etc).
+    /// Index matches `streams`. None for streams without codec init data.
+    pub codec_privates: Vec<Option<Vec<u8>>>,
 }
 
 /// A clip reference within a title.
@@ -797,6 +800,7 @@ impl DiscTitle {
             chapters: Vec::new(),
             extents: Vec::new(),
             content_format: ContentFormat::BdTs,
+            codec_privates: Vec::new(),
         }
     }
 
@@ -1502,6 +1506,7 @@ mod tests {
             chapters: Vec::new(),
             extents: Vec::new(),
             content_format: ContentFormat::BdTs,
+            codec_privates: Vec::new(),
         }
     }