Adds a Halt field to DiscStream, populated via the new
`with_halt(halt)` builder. The internal recovery / fill_extents
loops check `halt.is_cancelled()` directly. The existing
`set_halt(Arc<AtomicBool>)` method stays through the deprecation
window for callers (autorip mux) that haven't migrated; marked
#[deprecated] with a pointer to the constructor-time path.
Both signals are unified inside DiscStream: either Halt or the
legacy Arc<AtomicBool> triggers cancellation, so callers can mix
during the deprecation window without breaking stop behaviour.
See (internal)/memory/0_18_redesign.md and
0_18_round3_migration_audit.md.
Single contributor: MattJackson.
MattJackson
44e2c64d7e