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/mux/mkvstream.rs

@@ -254,11 +254,11 @@ impl MvcMerge {
                 .front()
                 .map(|pb| pb.additional.is_some())
                 .unwrap_or(false);
-            if front_ready || self.pending_base.len() > MVC_PAIR_WINDOW {
-                if let Some(pb) = self.pending_base.pop_front() {
-                    out.push((pb.frame, pb.additional));
-                    continue;
-                }
+            if (front_ready || self.pending_base.len() > MVC_PAIR_WINDOW)
+                && let Some(pb) = self.pending_base.pop_front()
+            {
+                out.push((pb.frame, pb.additional));
+                continue;
             }
             break;
         }