v0.20.3: add halt check to Disc::patch backtrack inner loop

WO-5 (partial): the patch backtrack inner loop ('while bt_pos <
backtrack_end' in disc/patch.rs) issues per-sector reads to fill the
gap created by a damage-window skip. A long backtrack span can run
minutes; without an inline halt poll, the outer halt only takes
effect when control returns to the per-range loop. Adds a halt poll
at the top of each iteration so cancellation propagates inside the
backtrack span.

Per-sector read failures inside the backtrack already drop through
to the main fail path; this only changes the cancellation latency
between an /api/stop call and the producer actually unwinding. Drops
worst-case unwind from 'whole backtrack span × per-sector recovery
timeout' (minutes) to 'one in-flight SCSI command' (seconds).

The broader Arc<AtomicBool> → Halt migration on CopyOptions /
SweepOptions / PatchOptions / Drive::halt and Pipeline::send halt-
awareness is deferred — separate cycle, larger API impact.

src/disc/patch.rs

@@ -895,6 +895,18 @@ impl Disc {
                                 );
                                 let mut bt_pos = backtrack_start;
                                 while bt_pos < backtrack_end {
+                                    // Honor cancellation inside the
+                                    // backtrack inner loop. A long
+                                    // backtrack span can run minutes
+                                    // of single-sector reads; without
+                                    // this check the outer halt only
+                                    // takes effect when control
+                                    // returns to the per-range loop.
+                                    if let Some(h) = &opts.halt {
+                                        if h.load(std::sync::atomic::Ordering::Relaxed) {
+                                            return Err(crate::error::Error::Halted);
+                                        }
+                                    }
                                     let span =
                                         // Backtrack always at count=1: this path
                                         // fills a gap that the main loop's damage-