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.
MattJackson
2667d68675