v0.20.0: delete FrameSource/FrameSink, keep single Stream trait

The 0.18 trait split into FrameSource (read-only) and FrameSink
(write-only) was an over-engineered API. Consumers don't think
"frame source backed by MKV" — they think "open MKV for reading".
The split paid a real API-complexity cost (two trait names, two
re-exports, dual impls per bidirectional type, deprecation bridge)
for one marginal property: compile-time direction-safety at the
trait-object boundary. The runtime error path on a wrong-direction
call (StreamReadOnly / StreamWriteOnly) is unambiguous and rare in
practice.

Deletions:
- pes::Stream is no longer #[deprecated]
- pes::FrameSource trait + its blanket-from-Stream bridge
- pes::FrameSink trait + the trampoline impls on every concrete type
- The compile-time-direction-safety test scaffolding
- Crate-root FrameSource / FrameSink re-exports

Additions:
- Stream is now Send-bounded (Stream: Send supertrait). Every
  concrete impl was already Send-compliant — Box<dyn Read + Send>
  and Box<dyn Write + Send> were already in place on the trait
  objects MkvStream / M2tsStream / etc hold internally. Promoting
  Send into the trait makes Box<dyn Stream> Send too, which lets
  autorip drop its SendStream unsafe newtype.

The public API is now: one Stream trait, one concrete type per
format, two constructors (open/create or input/output). Bidirectional
types route through internal Mode { Read | Write } discriminants.

Net: -347 lines libfreemkv, -38 lines autorip, -5 lines freemkv.

src/mux/mkvstream.rs

@@ -92,7 +92,6 @@ impl MkvStream {
     }
 }
 
-#[allow(deprecated)] // 0.18 trait split: migrate to FrameSource/FrameSink in follow-up commit.
 impl crate::pes::Stream for MkvStream {
     fn read(&mut self) -> io::Result<Option<crate::pes::PesFrame>> {
         let rs = match self.mode {
@@ -189,35 +188,6 @@ impl crate::pes::Stream for MkvStream {
     }
 }
 
-/// FrameSink sibling to the deprecated Stream impl; both coexist during the
-/// 0.18 deprecation window. Caller may pick either at the trait-object
-/// boundary — `Box<dyn Stream>` (deprecated) or `Box<dyn FrameSink>` (new).
-/// Use `MkvStream::create(writer, title)` to construct the write half;
-/// calling `FrameSink::write` on a `MkvStream::open(reader)` instance returns
-/// `StreamReadOnly`. `finish` is where the Cues index is written, so it must
-/// be called for the resulting MKV to be seekable.
-#[allow(deprecated)] // delegating to deprecated Stream during the 0.18 deprecation window so callers don't see the deprecation twice.
-impl crate::pes::FrameSink for MkvStream {
-    fn write(&mut self, frame: &crate::pes::PesFrame) -> io::Result<()> {
-        <Self as crate::pes::Stream>::write(self, frame)
-    }
-
-    fn finish(self: Box<Self>) -> io::Result<()> {
-        // Why: Stream::finish takes &mut self, FrameSink::finish takes Box<Self>.
-        // Re-borrow inside the box, call Stream::finish, drop the box.
-        // The inner `MkvMuxer` is owned via `Option`, and `Stream::finish`
-        // already takes it via `Option::take()` — moving `*self` out of the
-        // box hands it the same field by-value, so the muxer's own
-        // by-value `finish()` runs correctly.
-        let mut s: Self = *self;
-        <Self as crate::pes::Stream>::finish(&mut s)
-    }
-
-    fn info(&self) -> &crate::disc::DiscTitle {
-        <Self as crate::pes::Stream>::info(self)
-    }
-}
-
 // ── MKV header parsing (read side) ────────────────────────────
 
 /// Returns (DiscTitle, codec_privates: Vec<(track_number, codec_private_bytes)>)