v0.20.0: delete FrameSource/FrameSink, keep single Stream trait

The 0.18 trait split into FrameSource (read-only) and FrameSink
(write-only) was an over-engineered API. Consumers don't think
"frame source backed by MKV" — they think "open MKV for reading".
The split paid a real API-complexity cost (two trait names, two
re-exports, dual impls per bidirectional type, deprecation bridge)
for one marginal property: compile-time direction-safety at the
trait-object boundary. The runtime error path on a wrong-direction
call (StreamReadOnly / StreamWriteOnly) is unambiguous and rare in
practice.

Deletions:
- pes::Stream is no longer #[deprecated]
- pes::FrameSource trait + its blanket-from-Stream bridge
- pes::FrameSink trait + the trampoline impls on every concrete type
- The compile-time-direction-safety test scaffolding
- Crate-root FrameSource / FrameSink re-exports

Additions:
- Stream is now Send-bounded (Stream: Send supertrait). Every
  concrete impl was already Send-compliant — Box<dyn Read + Send>
  and Box<dyn Write + Send> were already in place on the trait
  objects MkvStream / M2tsStream / etc hold internally. Promoting
  Send into the trait makes Box<dyn Stream> Send too, which lets
  autorip drop its SendStream unsafe newtype.

The public API is now: one Stream trait, one concrete type per
format, two constructors (open/create or input/output). Bidirectional
types route through internal Mode { Read | Write } discriminants.

Net: -347 lines libfreemkv, -38 lines autorip, -5 lines freemkv.

tests/integration_progress_and_halt.rs

@@ -1,11 +1,6 @@
 //! 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;

tests/streams.rs

@@ -1,10 +1,5 @@
 //! Integration tests for the PES stream pipeline.
 
-// 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::mux::meta::M2tsMeta;
 use libfreemkv::pes::Stream as PesStream;
 use libfreemkv::*;