mux: PrefetchedSectorSource event_fn + delete DiscStream::new_pipeline

* `PrefetchedSectorSource::new_with_events` adds an optional
  `event_fn` callback that fires `BytesRead` after every successful
  batch from the producer thread. The original `new()` becomes a
  thin no-events wrapper. Lets autorip wire the highway and still
  get UI progress events without polling the consumer side.

* `build_iso_pipeline` grows an `event_fn` arg so the autorip
  multipass mux can pipe BytesRead straight through to its progress
  UI.

* Stream trait gains a default `errors() -> u64` method (= 0) so
  Box<dyn Stream> callers (autorip's mux loop) can read the
  skip-on-error counter without downcasting. `DiscStream` overrides
  to return its `errors` field.

* Delete `DiscStream::new_pipeline` and the pipeline-mode fields
  (`demux_thread`, `demux_rx`) plus the `read_pipeline` helper.
  All pipeline construction now goes through
  `PipelinedPesStream` via `build_iso_pipeline`; `DiscStream`
  becomes the single-thread-only inline path used by the drive
  single-pass read.

* `lib.rs` re-exports `build_iso_pipeline`.

src/lib.rs

@@ -211,6 +211,7 @@ pub use mux::{InputOptions, StreamUrl, input, output, parse_url};
 // are the ISO-on-disk implementations. [`DecryptingSectorSource`] is the
 // single decrypt-on-read decorator (AACS / CSS / none) — wrap any
 // `SectorSource` to get plaintext sectors out.
+pub use mux::build_iso_pipeline;
 pub use scsi::{DriveInfo, ScsiSense, ScsiTransport, drive_has_disc, list_drives};
 pub use sector::{
     DecryptingSectorSource, FileSectorSink, FileSectorSource, PrefetchedSectorSource, SectorSink,