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.

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@1.87.0
+      - uses: dtolnay/rust-toolchain@1.97.0
         with:
           components: clippy, rustfmt
       - uses: Swatinem/rust-cache@v2
@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@1.87.0
+      - uses: dtolnay/rust-toolchain@1.97.0
       - uses: Swatinem/rust-cache@v2
       - run: cargo test --tests
 
@@ -33,7 +33,7 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@1.87.0
+      - uses: dtolnay/rust-toolchain@1.97.0
       - uses: Swatinem/rust-cache@v2
       - run: cargo check
 
@@ -41,7 +41,7 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@1.87.0
+      - uses: dtolnay/rust-toolchain@1.97.0
       - uses: Swatinem/rust-cache@v2
       # Build the tests (not just `cargo check`): catches errors in test
       # code and forces full codegen of the Windows-only SPTI transport