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/mux/resolve.rs

@@ -234,6 +234,7 @@ pub fn input(url: &str, opts: &InputOptions) -> io::Result<Box<dyn crate::pes::S
                 ISO_MUX_BATCH_SECTORS,
                 format,
                 None,
+                None,
             );
             Ok(Box::new(stream))
         }
@@ -357,15 +358,17 @@ pub fn build_iso_pipeline<S: SectorSource + Send + 'static>(
     batch_sectors: u16,
     format: ContentFormat,
     halt: Option<crate::halt::Halt>,
+    event_fn: Option<crate::sector::prefetched::EventFn>,
 ) -> PipelinedPesStream {
     let extents = title.extents.clone();
     let decrypting =
         crate::sector::DecryptingSectorSource::new(Box::new(reader) as Box<dyn SectorSource>, keys);
-    let prefetched = crate::sector::PrefetchedSectorSource::new(
+    let prefetched = crate::sector::PrefetchedSectorSource::new_with_events(
         decrypting,
         extents,
         batch_sectors,
         halt.clone(),
+        event_fn,
     );
     let (rx, recycle_tx, shell) = prefetched.into_channels();