Bisect: halve at dead boundary + exempt boundary probes from stall

The expand loops broke out of the batch sweep on the first failing read,
overshooting the good/dead boundary by up to a batch and leaving the
readable sectors adjacent to a dead edge for the re-bisect to re-pin. On
top of that, early-yield counted those boundary-probe failures as a
stall, so the forward expand quit early AND the backward expand inherited
a poisoned unproductive counter and yielded instantly.

Now on a failed expand batch we halve the step (down to a single sector)
to recover right up to the dead edge in ~log2(batch) reads, drive the
expand loops off the deadline only (they self-terminate via halving), and
clear the unproductive streak once an island is located. patch_recovers_
good_middle_of_a_bad_range now recovers 50/50 good-middle sectors.

src/disc/patch.rs

@@ -912,6 +912,7 @@ impl PatchCtx<'_, '_> {
                 halt: self.opts.halt.as_deref(),
                 decrypt_is_aacs: self.decrypt_is_aacs,
                 tick: Some(&mut tick),
+                unproductive: 0,
             };
             run_handlers(&mut ctx, &mut handlers, bad, &mut self.scoreboard, |_bad| {
                 now_ptr() + std::time::Duration::from_secs(PER_HANDLER_BUDGET_SECS)