Add an advisory CI job on latest stable

Pinning CI meant a future Rust release could break the build without
anyone noticing until the pin moved. This job tracks latest stable and
runs the test suite and clippy, so that breakage is visible early.

Marked `continue-on-error`, so it reports rather than gates: a new
clippy lint landing in stable should not fail a pull request that did
not cause it. The pinned `test` job remains the gate.

Note that GitHub renders a `continue-on-error` job as passing with an
annotation rather than as failed. If it should be more visible, make it
a required check and drop the flag.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -34,3 +34,19 @@ jobs:
       # and pinning those to the oldest supported toolchain is not useful.
       - uses: dtolnay/rust-toolchain@1.88.0
       - run: cargo test
+
+  stable:
+    name: Latest stable (advisory)
+    runs-on: ubuntu-latest
+    # Advisory only. The pinned `test` job is the gate; this one exists so
+    # that a new stable release breaking the build or firing a new clippy
+    # lint is visible before the pin moves, without failing a pull request
+    # that did not cause it. Check it when bumping `devVersion`.
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy
+      - run: cargo test
+      - run: cargo clippy --all-targets -- -D warnings