chore: pin rust toolchain to 1.99 instead of floating stable

.github/workflows/ci.yml

@@ -53,7 +53,9 @@ jobs:
         with:
           toolchain: "1.88"
       - uses: Swatinem/rust-cache@v2
-      - run: cargo check --workspace --all-features --all-targets
+      # `+1.88` is explicit: rust-toolchain.toml would otherwise override the
+      # toolchain installed above and this job would silently check on 1.99.
+      - run: cargo +1.88 check --workspace --all-features --all-targets
 
   test:
     name: test

rust-toolchain.toml

@@ -0,0 +1,11 @@
+# Pinned to a stable minor rather than floating on `stable`: a new stable
+# ships new clippy lints, and with `-D warnings` that turned main red the day
+# Rust 1.99 released (#43). Patch releases (1.99.x) still float in.
+#
+# Bump deliberately in its own PR: change the version, run
+# `cargo clippy --workspace --all-targets --all-features -- -D warnings`, fix
+# the new lints. The MSRV job runs `cargo +1.88` explicitly, so this file does
+# not override it.
+[toolchain]
+channel = "1.99"
+components = ["rustfmt", "clippy"]