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.

tests/integration_progress_and_halt.rs

@@ -1,6 +1,11 @@
 //! Integration tests for progress reporting, halt behavior, drop safety,
 //! and the file-backed sector reader round trip.
 
+// 0.18 trait split: this suite still drives the deprecated `pes::Stream`
+// trait directly. It will be migrated to `FrameSource`/`FrameSink` in the
+// follow-up that ports concrete impls.
+#![allow(deprecated)]
+
 use libfreemkv::disc::{CopyOptions, DiscRegion};
 use libfreemkv::error::Result;
 use libfreemkv::pes::Stream as PesStream;