v0.17.11: sweep producer/consumer split — overlap drive read with file write

Pre-0.17.11 sweep ran strictly serialised: SCSI read → decrypt → seek
+ write → mapfile.record → next read. Drive idled for the post-read
work; throughput capped at the sum of both costs. On a healthy disc
that's ~7-12 ms read + ~5-15 ms write/record per 64 KB batch, limiting
sustained throughput to ~10-12 MB/s on the test bed (BU40N + UHD inner
zone), well below the ~14-16 MB/s drive ceiling.

Decouples them: producer thread (caller's) owns SectorReader +
read_error state + decrypt + set_speed + halt; consumer thread (one
spawn) owns Writer + Mapfile, receives WorkItem messages, applies
file write + mapfile record. Bounded mpsc::sync_channel(4) gives
natural back-pressure. While the consumer writes batch N, the
producer is already reading batch N+1 — steady-state throughput is
now bound by the slower of the two pipelines (drive on healthy
discs), not their sum.

Side effects:
- Bisect path now decrypts. Pre-0.17.11 the bisect inner loop wrote
  raw cyphertext for single-sector recoveries on encrypted discs —
  quiet correctness bug exercised only by batch-fail-then-
  bisect-succeed on encrypted media. New producer-side decrypt
  covers main + bisect success paths uniformly.
- All read_ctx state stays single-threaded on producer (damage
  window, jump multiplier, etc.). No locking added.
- Mapfile remains single-writer on consumer. No locking.
- Halt latency: producer breaks loop, sends Finish, consumer drains
  ≤4 in-flight items + sync_all. ~1 batch (~12 ms) typical.
- BU40N + Initio bridge wedge concern unchanged: still single SCSI
  command in flight, error-path timing identical, no new retries.

New module: src/disc/sweep_pipeline.rs (WorkItem, ProgressSnapshot,
ConsumerInputs, spawn_consumer, consumer_loop, helpers). Public API
unchanged — Disc::copy / CopyOptions / CopyResult identical.

Patch (Pass N) is NOT changed; it's bound by drive recovery time, not
the read/write serialisation.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libfreemkv"
-version = "0.17.10"
+version = "0.17.11"
 edition = "2024"
 rust-version = "1.86"
 license = "AGPL-3.0-only"