v0.12.2: hide halt behind checked_sleep / checked_exec primitives

Drive::read is now halt-check-free in its body. Previously, the halt
flag was checked in 4 places and the sleep logic was scattered across
4 "if halt_aware_sleep { return Halted }" call sites — correct, but
the ugliness invited drift: a new sleep added by someone unfamiliar
with the pattern would silently swallow Stop requests.

Two private primitives now own halt awareness:

  checked_sleep(Duration) -> Result<()>
  checked_exec(cdb, dir, buf, timeout_ms) -> Result<ScsiResult>

Both return Err(Halted) instead of a bool. The ? operator in read()
then propagates halts for free. The recovery path reads top-to-bottom
with no halt vocabulary.

sleep_until_halted lives as a free function so it's unit-testable
without a live Drive. 4 new tests: completes normally, bails on
pre-set flag within one slice, wakes mid-sleep, zero-duration no-op.

Public API unchanged — halt_flag/halt/clear_halt still exposed, the
refactor is entirely internal.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libfreemkv"
-version = "0.12.1"
+version = "0.12.2"
 edition = "2024"
 rust-version = "1.86"
 license = "AGPL-3.0-only"