Sweep the pinned toolchain to Rust 1.97

The Windows UI needs current winsafe, whose real minimum is 1.89 (its manifest
under-declares 1.87 while it uses NonNull::from_ref). Rather than stop at the
minimum, this goes to current stable and fixes what that costs.

The counter-intuitive result: 1.97 is CHEAPER than 1.89. libfreemkv had 54
clippy errors at 1.89 and 6 at 1.97, because clippy tightened the noisy
collapsible_if lint in between. Stopping at the minimum would have been the most
expensive choice available.

Roughly 47 lints across the eight repos, the large majority auto-fixed:
libfreemkv 6, freemkv-engine 14, bdemu 8, freemkv-keysources 7, autorip 6,
freemkv-unlock 3, freemkv-i18n 3. The hand-fixed ones are a descending sort to
sort_by_key(Reverse), four manual checked-division sites, a loop counter replaced
by enumerate, and a loop whose first let-else became a while-let.

Worth recording for whoever bumps next: clippy is MSRV-AWARE. Those 54 lints only
appear once the crate DECLARES 1.89 or later, because let-chains become
available. A bare `cargo +1.89 clippy` against a manifest still pinned at 1.87
reports clean and is meaningless — gate with the real precommit script, which is
also the only thing that covers build scripts.

The pin still sits below the Mac default, so it keeps doing its job: catching
lint drift locally before CI sees it.

src/sector/prefetched.rs

@@ -227,7 +227,9 @@ impl PrefetchedSectorSource {
                         // way to hand the decrypt step an aligned chunk —
                         // surface a typed error instead of emitting
                         // still-encrypted bytes.
-                        if remaining % unit_align as u32 != 0 && remaining < unit_align as u32 {
+                        if !remaining.is_multiple_of(unit_align as u32)
+                            && remaining < unit_align as u32
+                        {
                             let _ = tx.send(Err(crate::error::Error::ExtentNotUnitAligned.into()));
                             return;
                         }