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/disc/dvd_audio_probe.rs

@@ -104,10 +104,10 @@ fn max_substream_channels(data: &[u8]) -> Option<u8> {
         };
         let start = pos + rel;
         let frame = &data[start..];
-        if let Some(ch) = ac3::acmod_channels(frame) {
-            if ch > 0 {
-                best = Some(best.map_or(ch, |b| b.max(ch)));
-            }
+        if let Some(ch) = ac3::acmod_channels(frame)
+            && ch > 0
+        {
+            best = Some(best.map_or(ch, |b| b.max(ch)));
         }
         // Advance past this frame by its declared size when that is mappable;
         // otherwise step 2 bytes past the sync and re-scan for the next one.