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/null.rs

@@ -16,7 +16,6 @@ impl NullStream {
     }
 }
 
-#[allow(deprecated)] // 0.18 trait split: migrate to FrameSink in follow-up commit.
 impl crate::pes::Stream for NullStream {
     fn read(&mut self) -> io::Result<Option<crate::pes::PesFrame>> {
         Ok(None)
@@ -32,43 +31,16 @@ impl crate::pes::Stream for NullStream {
     }
 }
 
-/// 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).
-/// The deprecation-window callers eventually migrate; this impl exists so
-/// new callers can target `FrameSink` without waiting for the rest of the
-/// migration to complete.
-#[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 NullStream {
-    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.
-        let mut s: Self = *self;
-        <Self as crate::pes::Stream>::finish(&mut s)
-    }
-
-    fn info(&self) -> &DiscTitle {
-        <Self as crate::pes::Stream>::info(self)
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::pes::FrameSink;
+    use crate::pes::Stream;
 
-    /// Smallest credible witness that the new FrameSink impl on a concrete
-    /// `mux/*` sink works through the trait object: build a boxed
-    /// `dyn FrameSink`, write a frame, finish it. The trait-bridge correctness
-    /// is what's being verified — not NullStream-specific behaviour.
+    /// Verify NullStream routes through the `Stream` trait object cleanly.
     #[test]
-    fn frame_sink_via_dyn_object_writes_and_finishes() {
+    fn stream_via_dyn_object_writes_and_finishes() {
         let title = DiscTitle::empty();
-        let mut sink: Box<dyn FrameSink> = Box::new(NullStream::new(&title));
+        let mut sink: Box<dyn Stream> = Box::new(NullStream::new(&title));
 
         let frame = crate::pes::PesFrame {
             track: 0,
@@ -77,9 +49,7 @@ mod tests {
             data: vec![0x01, 0x02, 0x03],
         };
         sink.write(&frame).unwrap();
-        // info() routes through the trait object.
         let _ = sink.info();
-        // finish() consumes the Box<Self> — must compile and run.
         sink.finish().unwrap();
     }
 }