0.18 primitive: FrameSource/FrameSink trait split (deprecate Stream)

Splits the bidirectional pes::Stream into one-direction traits so
calling read() on a write-only sink is a compile error instead of
runtime E9001. Keeps Stream alive as deprecated through 0.18 with a
blanket FrameSource impl so existing concrete types compile unchanged.

FrameSink can't be blanket-impl'd from Stream (different finish
signature), so concrete impls migrate per-type in a follow-up.

Concrete `impl pes::Stream for X` blocks in mux/* and the existing
tests gain a one-line `#[allow(deprecated)]` to keep `-D warnings`
clean during the deprecation window — no behavior changes.

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

Single contributor: MattJackson.

src/mux/disc.rs

@@ -347,6 +347,7 @@ impl DiscStream {
     }
 }
 
+#[allow(deprecated)] // 0.18 trait split: migrate to FrameSource/FrameSink in follow-up commit.
 impl crate::pes::Stream for DiscStream {
     fn read(&mut self) -> io::Result<Option<crate::pes::PesFrame>> {
         if let Some(frame) = self.pending_frames.pop_front() {

src/mux/m2ts.rs

@@ -173,6 +173,7 @@ impl M2tsStream {
     }
 }
 
+#[allow(deprecated)] // 0.18 trait split: migrate to FrameSource/FrameSink in follow-up commit.
 impl crate::pes::Stream for M2tsStream {
     fn read(&mut self) -> io::Result<Option<crate::pes::PesFrame>> {
         if let Some(frame) = self.pending_frames.pop_front() {

src/mux/mkvstream.rs

@@ -92,6 +92,7 @@ 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 {

src/mux/network.rs

@@ -70,6 +70,7 @@ impl NetworkStream {
     }
 }
 
+#[allow(deprecated)] // 0.18 trait split: migrate to FrameSource/FrameSink in follow-up commit.
 impl crate::pes::Stream for NetworkStream {
     fn read(&mut self) -> io::Result<Option<crate::pes::PesFrame>> {
         match &mut self.mode {
@@ -157,6 +158,7 @@ mod tests {
 
     #[test]
     #[ignore] // Requires TCP; may be flaky in CI environments
+    #[allow(deprecated)] // 0.18 trait split: migrate to FrameSource/FrameSink in follow-up commit.
     fn network_pes_roundtrip() {
         use crate::pes;
 

src/mux/null.rs

@@ -16,6 +16,7 @@ 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)

src/mux/resolve.rs

@@ -167,6 +167,7 @@ pub struct InputOptions {
 }
 
 /// Open a PES input stream (produces PES frames).
+#[allow(deprecated)] // 0.18 trait split: migrate to FrameSource in follow-up commit.
 pub fn input(url: &str, opts: &InputOptions) -> io::Result<Box<dyn crate::pes::Stream>> {
     let parsed = parse_url(url);
     match parsed {
@@ -234,6 +235,7 @@ pub fn input(url: &str, opts: &InputOptions) -> io::Result<Box<dyn crate::pes::S
 }
 
 /// Open a PES output stream (consumes PES frames).
+#[allow(deprecated)] // 0.18 trait split: migrate to FrameSink in follow-up commit.
 pub fn output(
     url: &str,
     title: &crate::disc::DiscTitle,

src/mux/stdio.rs

@@ -60,6 +60,7 @@ impl StdioStream {
     }
 }
 
+#[allow(deprecated)] // 0.18 trait split: migrate to FrameSource/FrameSink in follow-up commit.
 impl crate::pes::Stream for StdioStream {
     fn read(&mut self) -> io::Result<Option<crate::pes::PesFrame>> {
         self.ensure_header_read()?;