0.18 round 2: add FrameSink impls to concrete mux sinks

Per-impl migration of MkvStream / M2tsStream / NetworkStream /
NullStream / StdioStream from the deprecated pes::Stream trait
to the typed pes::FrameSink trait. Both impls coexist during
the 0.18 deprecation window — the existing Stream impls are
unchanged.

The FrameSink::finish signature differs (Box<Self> vs &mut self),
which is why this couldn't be a blanket impl. Each migration
re-borrows the box and delegates to the underlying Stream::finish
body.

FrameSink: Send forced two struct fields (M2tsStream's boxed
Write/Read, MkvStream's boxed WriteSeek/Read) to gain `+ Send`
bounds — minimum surface needed to make the Send-bounded trait
impl-able. mux::resolve::output's local Box<dyn WriteSeek>
construction picks up the same `+ Send`. tests/streams.rs's
shared `stream.write/.finish/.info/.read` calls were
disambiguated to `PesStream::*` to resolve the now-multiple
candidates from coexisting trait impls.

Caller migration (mux::resolve::output return type, autorip,
CLI) is a later slice. This commit only adds new impls; nothing
removed.

See (internal)/memory/0_18_redesign.md.

Single contributor: MattJackson.

src/mux/null.rs

@@ -31,3 +31,55 @@ impl crate::pes::Stream for NullStream {
         &self.disc_title
     }
 }
+
+/// 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;
+
+    /// 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.
+    #[test]
+    fn frame_sink_via_dyn_object_writes_and_finishes() {
+        let title = DiscTitle::empty();
+        let mut sink: Box<dyn FrameSink> = Box::new(NullStream::new(&title));
+
+        let frame = crate::pes::PesFrame {
+            track: 0,
+            pts: 0,
+            keyframe: true,
+            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();
+    }
+}