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/labels/png_filenames.rs

@@ -41,10 +41,10 @@ pub fn parse(_reader: &mut dyn SectorSource, udf: &UdfFs) -> Option<ParseResult>
 fn labels_from_filenames(names: &[String]) -> Vec<StreamLabel> {
     let mut seen: Vec<&'static str> = Vec::new();
     for name in names {
-        if let Some(code) = filename_lang(name) {
-            if !seen.contains(&code) {
-                seen.push(code);
-            }
+        if let Some(code) = filename_lang(name)
+            && !seen.contains(&code)
+        {
+            seen.push(code);
         }
     }
     seen.into_iter()