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 (internal)/memory/0_18_redesign.md.

Single contributor: MattJackson.

docs/udf.md

@@ -126,7 +126,7 @@ USB optical drives have ~500ms round-trip latency per SCSI command. Since `read_
 
 `Disc::scan()` wraps the drive in a `BufferedSectorReader` before reading. On a single-sector read, the buffer prefetches a batch of sectors (sized from the kernel's `max_hw_sectors_kb` for the device) and caches them. Subsequent reads to nearby LBAs return from cache with zero SCSI overhead. After parsing the UDF directory structure, the entire metadata partition is pre-read into the cache, so all ICB lookups during title scanning and encryption resolution are instant.
 
-The buffer is transparent -- `read_filesystem()`, `read_file()`, and all downstream code still call `read_sectors(lba, 1, buf)` as before. The batching happens inside the `SectorReader` implementation.
+The buffer is transparent -- `read_filesystem()`, `read_file()`, and all downstream code still call `read_sectors(lba, 1, buf)` as before. The batching happens inside the `SectorSource` implementation.
 
 ### UDF Filename Encoding