0.18.1 docs: refresh README, CHANGELOG, and docs/ for the trait split

The library's public-facing docs were sitting on the 0.17 trait
surface — Disc::copy, pes::Stream, SectorReader, etc. — even though
all in-tree callers migrated in 0.18 rounds 1-3. With 0.18.1 about
to ship, a user copy-pasting the README sample from crates.io would
have hit a compile error.

This commit is purely doc-side:

- README.md: Quick Start rewritten onto Disc::sweep + Disc::patch
  with caller-orchestrated multipass; Streams table footnote and
  Architecture row reference FrameSource / FrameSink.
- CHANGELOG.md: 0.18.1 entry describing the redesign — primitives,
  trait splits, deprecations (kept alive through 0.18.x, deletion
  target 0.18.2), throughput numbers.
- docs/{rip-recovery,api-design,architecture,disc-to-rip,
  drive-access,udf}.md: every Disc::copy / pes::Stream /
  SectorReader reference updated to the 0.18 trait surface.
- FEATURES.md: deleted (8+ versions stale; capabilities live in
  README.md and CHANGELOG.md now, matching the workspace-top
  FEATURES.md removal in 84acd65).
- examples/iso_dump.rs: verified compiles against 0.18.1.

No code changes.

See freemkv-private/memory/0_18_redesign.md.

Single contributor: MattJackson.

docs/architecture.md

@@ -65,8 +65,12 @@ libfreemkv (lib.rs)
 │
 ├── Streaming
 │   ├── mux/          Stream implementations (Disc, ISO, MKV, M2TS, Network, Stdio, Null)
-│   ├── pes           PES frame types, Stream trait (read/write frames)
-│   └── sector        SectorReader trait — abstracts disc vs ISO vs file
+│   ├── pes           PES frame types; FrameSource / FrameSink direction-typed traits
+│   └── sector/       SectorSource / SectorSink traits, FileSector{Source,Sink}, DecryptingSectorSource
+│
+├── I/O Primitives
+│   ├── halt          Halt cancellation token (one Arc<AtomicBool>, cloneable)
+│   └── io/           Pipeline<I, R> + Sink trait + WritebackFile (bounded-cache writer)
 │
 ├── Support
 │   ├── keydb         KEYDB.cfg download, parse, verify, save