Fix CI: use stable toolchain for rustfmt

Author: silva-fj

ci.sh

@@ -5,8 +5,8 @@ set -e
 # Build
 cargo build
 
-# Check formatting
-cargo fmt -- --check
+# Check formatting (use stable toolchain explicitly)
+cargo +stable fmt -- --check
 
 # Run Clippy
 cargo clippy -- -D warnings