ci: test the no-alloc build and harden the workflow

Run clippy, docs and tests for tines-core with --no-default-features,
gating the MemStore-based tests on `alloc` so they compile there. Fail on
rustdoc warnings, pass --locked everywhere, and pin the toolchain to
1.99.0. In the workflow, use read-only permissions, cancel superseded
runs, add a 20-minute timeout, and pin actions to commit SHAs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -5,24 +5,34 @@ on:
     branches: [master]
   pull_request:
 
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   CARGO_TERM_COLOR: always
 
 jobs:
   ci:
     runs-on: ubuntu-latest
+    timeout-minutes: 20
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
 
+      # Keep in sync with rust-toolchain.toml.
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@683b588c487a69d01f6c6ddf40f036ff682cc2c8 # master
         with:
+          toolchain: 1.99.0
           components: rustfmt, clippy
           targets: thumbv7em-none-eabihf
 
       - name: Cache cargo registry and target
-        uses: Swatinem/rust-cache@v2
+        uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
 
       - name: Run CI gate
         run: sh scripts/ci.sh