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/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;