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.

build.rs

@@ -77,10 +77,10 @@ fn emit_git_suffix() {
     // Re-run when HEAD (or the branch it points at) moves so the stamp stays
     // current without a clean rebuild.
     println!("cargo:rerun-if-changed=.git/HEAD");
-    if let Ok(head) = std::fs::read_to_string(".git/HEAD") {
-        if let Some(ref_path) = head.strip_prefix("ref: ") {
-            println!("cargo:rerun-if-changed=.git/{}", ref_path.trim());
-        }
+    if let Ok(head) = std::fs::read_to_string(".git/HEAD")
+        && let Some(ref_path) = head.strip_prefix("ref: ")
+    {
+        println!("cargo:rerun-if-changed=.git/{}", ref_path.trim());
     }
 }