Add cargo fmt --check to CI and local CI

.github/workflows/ci.yml

@@ -27,6 +27,16 @@ env:
 # separate ci-compat.yml workflow so their failures don't mark this run red.
 # ─────────────────────────────────────────────────────────────────────────────
 jobs:
+  fmt:
+    name: Format check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
+      - run: cargo fmt --check
+
   build:
     name: Build (${{ matrix.os }})
     runs-on: ${{ matrix.os }}

testing/ci-local.sh

@@ -153,6 +153,14 @@ run_build() {
         return 1
     fi
 
+    info "cargo fmt --check"
+    if cargo fmt --check 2>&1; then
+        record "fmt" 0
+    else
+        record "fmt" 1
+        return 1
+    fi
+
     info "cargo clippy --all -- -D warnings"
     if cargo clippy --all -- -D warnings 2>&1; then
         record "clippy" 0