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/timeline.rs

@@ -140,14 +140,14 @@ impl TimelineContinuity {
             // overflow: a panic out of the public `Stream::write` path in an
             // overflow-checked build, and in release a wrap to the opposite sign
             // that fires the straggler clamp on essentially every passive frame.
-            if let Some(high) = self.high_ns {
-                if mapped > high.saturating_add(DISCONTINUITY_BACKSTEP_NS) {
-                    let prev_mapped = raw_pts_ns.saturating_add(self.prev_offset_ns);
-                    if prev_mapped <= high
-                        && prev_mapped >= high.saturating_sub(DISCONTINUITY_BACKSTEP_NS)
-                    {
-                        return prev_mapped;
-                    }
+            if let Some(high) = self.high_ns
+                && mapped > high.saturating_add(DISCONTINUITY_BACKSTEP_NS)
+            {
+                let prev_mapped = raw_pts_ns.saturating_add(self.prev_offset_ns);
+                if prev_mapped <= high
+                    && prev_mapped >= high.saturating_sub(DISCONTINUITY_BACKSTEP_NS)
+                {
+                    return prev_mapped;
                 }
             }
             return mapped;