0.18 round 2: refactor Disc::sweep onto Pipeline + SweepSink

Sweep was the original producer/consumer split that motivated the
generic Pipeline primitive (round 1, commit 500894e). Now that
Pipeline + Sink exist, sweep stops shipping its own bespoke
threading.

- New SweepSink: Sink<WorkItem> impl in src/disc/sweep.rs. Owns
  WritebackFile + Mapfile + ProgressSnapshot back-channel. apply()
  carries the file-write + mapfile.record per WorkItem; close()
  drains writeback, fsyncs, flushes mapfile.
- Disc::sweep: constructs SweepSink, calls Pipeline::spawn_named
  (so the consumer thread keeps showing up as
  freemkv-sweep-consumer), sends WorkItems, calls pipe.finish().
  The producer-side ReadCtx state machine, decrypt, set_speed,
  halt — all unchanged.
- Pipeline gains spawn_named(name, depth, sink) so callers can
  preserve identifiable thread names without the primitive baking
  one in. Also adds Pipeline::try_send for the throttled
  StatsRequest path that must not block the producer.
- Deleted src/disc/sweep_pipeline.rs entirely. WorkItem,
  ProgressSnapshot, ConsumerSummary moved into disc/sweep.rs as
  module-private types. WorkItem::Finish dropped — dropping the
  channel is the end-of-stream signal Pipeline already uses.

Behaviour-preserving: the sweep algorithm, mapfile invariants,
back-pressure via channel depth (DEFAULT_PIPELINE_DEPTH = 4) all
match the 0.17.13 implementation. New synthetic regression test
(sweep_pipeline_full_good_100_batches) exercises ~100 batches of
clean reads end-to-end through the new Pipeline path and verifies
bytes_good and ISO file size.

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

src/io/mod.rs

@@ -21,9 +21,11 @@ pub mod pipeline;
 
 pub(crate) use writeback_file::WritebackFile;
 
-// Re-exports for the 0.18 redesign. Currently flagged unused because
-// no in-tree call site has been migrated yet (sweep is still on
-// `disc/sweep_pipeline.rs`; patch and mux have no pipeline). The next
-// 0.18 slice removes this allow as it wires up the first consumer.
+// Re-exports for the 0.18 redesign. Sweep is wired up in
+// `disc/sweep.rs`; patch and mux migrate in later 0.18 slices.
+// `WRITE_THROUGH_DEPTH` is patch-only and currently has no in-tree
+// caller — the targeted `#[allow]` keeps the re-export visible
+// without dragging the rest of the module under `dead_code`.
 #[allow(unused_imports)]
-pub use pipeline::{DEFAULT_PIPELINE_DEPTH, Flow, Pipeline, Sink, WRITE_THROUGH_DEPTH};
+pub use pipeline::WRITE_THROUGH_DEPTH;
+pub use pipeline::{DEFAULT_PIPELINE_DEPTH, Flow, Pipeline, Sink};