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/stdio.rs

@@ -105,3 +105,26 @@ impl crate::pes::Stream for StdioStream {
         self.header_read || self.writer.is_some()
     }
 }
+
+/// 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 `StdioStream::output(title)` to construct the write half; calling
+/// `FrameSink::write` on a `StdioStream::input()` returns `StreamReadOnly`.
+#[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 StdioStream {
+    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)
+    }
+}